Steps I've tried
----------------------------------------
1) Remove battery.
2) Plug USB cable.
3) Insert Battery (while holding U or wihtout, same result).

Outcome
1) Nokia logo shows up (for about 30 seconds).
2) Phone reboots, and shows Nokia logo again. (Repeats about 5 times).
3) Stops rebooting.
4) Blank screen, and only shows steady glowing yellow notification light.
5) A few minutes later, goes back to 2).

----------------------------------------
1) Insert battery.
2) Plug USB cable.
3) Hold U (or without, same result).

Outcome
1) Blank screen, and only shows steady glowing yellow notification light.
2) A few minutes later, phone reboots, and shows Nokia logo again. (Repeats about 5 times).
3) Then goes back to 1).

==========================================
I can't seem to get the expected outcome (access to R&D mode) when plugging in the USB while pressing U button.

I have also tried to cold-flash it from both Vista and Ubuntu.

Procedure
1) Load up CMD or Terminal.
2) Remove battery.
3) Plug in USB cable.
4) Use cold flash mode:

Vista

C:\Users\User>cd\program files\maemo\flasher-3.5

C:\Program Files\maemo\flasher-3.5>flasher-3.5 -c -h RX-51:2101 -S usb -F RX-51_
2009SE_3.2010.02-8_PR_COMBINED_MR0_ARM.bin -f -R

or


C:\Users\User>cd\program files\maemo\flasher-3.5

C:\Program Files\maemo\flasher-3.5>flasher-3.5 -c -h RX-51:2201 -S usb -F RX-51_
2009SE_3.2010.02-8_PR_COMBINED_MR0_ARM.bin -f -R

Ubuntu
sudo flasher-3.5 -c -h RX-51:2101 -S usb -F '/...some directory/RX-51_2009SE_3.2010.02-8_PR_COMBINED_MR0_ARM.bin' -f -R

or

sudo flasher-3.5 -c -h RX-51:2201 -S usb -F '/...some directory/RX-51_2009SE_3.2010.02-8_PR_COMBINED_MR0_ARM.bin' -f -R

5) Insert battery.

==========================================
Outcome

Vista

flasher v2.5.2 (Sep 24 2009)

SW version in image: RX-51_2009SE_3.2010.02-8_PR_MR0
Image 'kernel', size 1704 kB
Version 2.6.28-20094803.3+0m5
Image 'rootfs', size 158336 kB
Version RX-51_2009SE_3.2010.02-8_PR_MR0
Image 'cmt-2nd', size 81408 bytes
Version BB5_09.36
Image 'cmt-algo', size 519808 bytes
Version BB5_09.36
Image 'cmt-mcusw', size 5824 kB
Version rx51_ICPR82_09w47
Image '2nd', size 14720 bytes
Valid for RX-51: 2217, 2218, 2219, 2220, 2120
Version 1.4.14.4.4+0m5
Image 'xloader', size 14848 bytes
Valid for RX-51: 2217, 2218, 2219, 2220, 2120
Version 1.4.14.4.4+0m5
Image 'secondary', size 109440 bytes
Valid for RX-51: 2217, 2218, 2219, 2220, 2120
Version 1.4.14.4.4+0m5
Image '2nd', size 14720 bytes
Valid for RX-51: 2101, 2102, 2103
Version 1.4.14.4.4+0m5
Image 'xloader', size 14848 bytes
Valid for RX-51: 2101, 2102, 2103
Version 1.4.14.4.4+0m5
Image 'secondary', size 109440 bytes
Valid for RX-51: 2101, 2102, 2103
Version 1.4.14.4.4+0m5
Image '2nd', size 14720 bytes
Valid for RX-51: 2104, 2105, 2106, 2107, 2108, 2109, 2110, 2111, 2112, 2
113, 2114, 2115, 2116, 2117, 2118, 2119
Version 1.4.14.4.4+0m5
Image 'xloader', size 14848 bytes
Valid for RX-51: 2104, 2105, 2106, 2107, 2108, 2109, 2110, 2111, 2112, 2
113, 2114, 2115, 2116, 2117, 2118, 2119
Version 1.4.14.4.4+0m5
Image 'secondary', size 109440 bytes
Valid for RX-51: 2104, 2105, 2106, 2107, 2108, 2109, 2110, 2111, 2112, 2
113, 2114, 2115, 2116, 2117, 2118, 2119
Version 1.4.14.4.4+0m5
Image '2nd', size 14720 bytes
Valid for RX-51: 2201, 2202, 2203, 2204, 2205, 2206, 2207, 2208, 2209, 2
210, 2211, 2212, 2213, 2214, 2215, 2216
Version 1.4.14.4.4+0m5
Image 'xloader', size 14848 bytes
Valid for RX-51: 2201, 2202, 2203, 2204, 2205, 2206, 2207, 2208, 2209, 2
210, 2211, 2212, 2213, 2214, 2215, 2216
Version 1.4.14.4.4+0m5
Image 'secondary', size 109440 bytes
Valid for RX-51: 2201, 2202, 2203, 2204, 2205, 2206, 2207, 2208, 2209, 2
210, 2211, 2212, 2213, 2214, 2215, 2216
Version 1.4.14.4.4+0m5
Using serial port usb
Suitable USB device not found, waiting.
USB device found found at bus bus-0, device address \\.\libusb0-0001--0x0421-0x0
106.
Waiting for ASIC id...
usb_bulk_read: Result too large

C:\Program Files\maemo\flasher-3.5>


Ubuntu

something numbers something something
numbers something numbers something
something numbers something something
ping......pong
ping......failed!


and then back to

Suitable USB device not found, waiting.


==============================================
What should I do now? :(

I think your battery is low. I had the same problem few weeks ago. Note: The Nokia 5800XM uses the same battery, so you can use it to charge or change the battery.

How do I know it is fully charged? The light stays yellow.

Also, have you read the instructions in the Wiki? The reason I ask is that you refer to removing the battery as if that is some sort of requirement, but the Wiki instructions I saw didn't say that.

Also, in the section saying you did it under Ubuntu, you say that you get the message

Suitable USB device not found, waiting.

and then ask what you should do then.

The Wiki specifically mentions that message and says what you should do then. If you read the instructions, why do you mention that message as if there are no further instructions at that point?

hahah i had EXACTLY the same problem, twice i freaked out hardcore both times, when its fully charged the light goes green, just let the phone charge for like an hour then try again!

What I have read is:
3. It will say:

Suitable USB device not found, waiting

* N900: Remove USB, hold the ‘u’ key and plug it back in. Wait until you get a boot screen with usb logo in top right, and let go of the key.

That's the part which I could not obtain.

It's also rebooting again and again. Is it normal?

Are you using the USB adapter that came with the N900 or another one? It matters.

Have you tried plugging in the USB adapter to a different USB port on the computer you are using?

I am focusing on the USB adapter and port because the only problem I see is that when you plug in the USB while holding down the u (flashing from Linux is the most reliable way to go, in my experience) you don't ever see the USB logo on the right upper part of your N900 screen. This indicates that the N900 never sees a USB connection.

When I did this, by the way, I found that the N900 screen was exceedingly dim at that point, so it would be easy not to see the USB logo even though it actually does appear.

That usually indicates that the phone is in a state where it cannot charge the battery. You'll need to charge it up externally (either in a compatible phone or in a desktop charger).

Rebooting again and again can definitely mean LOTS of things other than the battery can't be charged. In my case I had spare charged batteries and a desktop battery charger, so the rebooting didn't mean that.