1) Keyboard differences are a non-issue. If you prefer the new ALU keyboard, it costs $49 and works fine with white iMacs.
2) No one seems to have mentioned the biggest performance difference: SATA I vs. SATA II internal HD bus speeds. I do notice a slight performance penalty when browsing albums of large (3-4 MB) JPGs in iPhoto. Very minor, IMO, but real.
3) Also, the white 20" only has FW400, not FW800.
Nonetheless, after test-driving a 20" ALU and a 24" ALU for one month each, I gave up on the nasty, non-uniform displays and switched to a white 20", with an ACD-quality display.
No gradients, no freezing, no regrets.
BTW, gloss vs. matte was never an issue -- I had no preference when I bought the ALU 20" -- but in retrospect, after two months experience with gloss, I much prefer the matte display. YMMV.
Since I wanted a 20" from the start, I see the price difference as $1200 for 20" white versus $1800 for 24" ALU -- because even if Apple eventually fixes all the ALU display problems, the 20" ALU will still have a second-rate TN-film panel. Sorry, I refuse to pay $1500 for second-rate.
So, even if I could get a 24" ALU with an acceptable display, there's no way the 10% speed increase would justify a 50% price bump -- especially for computers that'll be seriously obsolete as soon as Apple introduces an iMac with BlueRay or HD DVD ...
... or a midi-tower with iMac-level performance,
LK