For someone coming from the N900, but who likes the iPhone as well I can recommend Android 2.2+ which has root, no locked bootloader, and community support.

It also needs a Cortex A8 clocked above 500MHz with over 400MB RAM. A slide out physical qwerty is a godsend, and you need the standard ports etc.

According to that criteria here is the list of all Android slide-qwerty phone contenders arranged from best->worst:
(choice depends on your gsm/cdma frequency, language and budget).

1) Motorola Droid3/Milestone 3 (locked)
2) Samsung EPIC 4G
3) HTC Desire Z / T-Mobile G2
4) HTC Evo Shift 4G
5) Sony Ericsson Xperia PRO (locked--unlockable)
6) Droid2WE / Milestone 2 (locked)
6) Droid (original OG)
7) Sony Ericsson Xperia Play (locked)
8) Cliq2 (locked)
9) Milestone (locked, but custom ROMs available)
10) Samsung Transform (BoostMobile)
11) Xperia Mini Pro SK17i (locked--unlockable)
12) Samsung M920
13) LG Optimus Q
14) MyTouch 3G Slide

-------vv beyond here, not worth it:
15) LG Ally / LG Axis / LG Apex / LG Aloha
16) Samsung Transform
17) Xperia X10 mini PRO (bootloader locked--bypassed!)
18) Motorola CLIQ / DEXT (750MHz + CM7 possible)
19) LG KH5200 Andro-1
20) Samsung Moment II / Samsung Intercept
21) The first consumer-Android device
HTC Dream / T-Mobile G1
22) Samsung Galaxy Indulge
23) Samsung Moment
24) Samsung Acclaim
25) LG GW620

These are the options, don't forget the Thanks button