Not the Nokia N900 - it needs recharging more thn once every 24 hours! Also if you look on eBay the N900 now sells for £150 2nd hand versus a new price of £500 - buyers don't like it. The iphone 3gs still makes about £250 - 300 on eBay - buyers do like it. I have just sent my old Nokia N95 to be repaired, its not quite a smartphone but very nearly. I paid £320 for it in 2007 - it would now fetch about £100 if still working, perhpas as much as £70 for parts. I recommend applying the resale value test to whatever phone you consider.
Andrew, whilst it's not perfect I'm a happy iphone 4 user. Bought for, and mainly used as, a work tool, but a good all-rounder. The camera's still not a patch on Nokia phone cameras though.