First things first: are you sure you have the D+ and D- pins connected in the right order?
Second, make sure you have the right pins soldered. The ASCII diagram that is in the wiki was kind of ambiguous to me, until I looked closer at a pic of the phone with the battery off.

http://cpkb.org/images/9/99/Nokia_n900_pinout.jpg
In this photo, you see the pinouts for the serial and Mbus port, but the USB pins are actually under the text "BSP" in the photo. Left is D+, right is D-.

To make sure you have all that connected right, hold U on the keyboard and plug in USB while the phone is off, then try turning it on while still holding U... if you're on windows, check the device manager- you may need a driver before you can reflash. If you're in linux, try running 'lsusb' and checking if the device shows up in the list.

If all that doesn't work, I'd suspect that it's either a soldering error or something that I wouldn't be able to tell without a few photos of the device.