I have an idea. An extreme idea, of course:
I have been having way too much fun with my jailbroken iPhone 3G. I bought it because I've had the fever ever since it came out, and finally had a reasonable justification for buying one: I'm co-designing an iPhone app and need to test it on a lowest common denominator system--the 3G, which is two years and two generations old.
I bought it for ~$220 and so far I'm not even using it as a phone, so there's no monthly cost to me; I just use wifi. It's in near mint condition, and I think I can keep it in that state for a while. Besides testing the app my cousins and I are designing, I have implemented a "Getting Things Done" system on Omnifocus, synced between the phone and my Mac--this is already paying dividends, frankly.
Anyway, it strikes me that Apple comes out with a new iPhone every June, like clockwork. The $220 I paid for this 3G is the most you'll ever pay for a 3G; the 3GS goes for about $100 more currently. I'm going to hypothesize that if I put the 3G on eBay in May 2011, I'll get what I paid for it back, and then if I wait 30 days I can get a 3GS for about the same amount as the iPhone 4 becomes "old news" and the 3GS becomes, "3G wha?"