Unlocking is the easy part. Getting your hands on a 3G iPhone that isn't activated and contracted to AT&T already, is the hard part.
Well, here's how I see it. This places the iPhone essentially in the same water as every other phone that AT&T sells, non? As well as every phone Verizon, Sprint, and T-Mobile sell. The only differences between all those phones and the iPhone are...
- The iPhone 3G officially requires a contract to activate even if you buy it used (probably addressed by unlocking it, a bit of a grey area, since there isn't an obvious legal basis for this requirement from AT&T when there is no similar policy for any other phone you buy used)
- The iPhone has a lot more buzz than the other phones.
So my suspicion is that, for the next 6-8 months, yes, it will be very hard to get one. After that, it'll be like other phones -- you can get pretty much any phone released in the US for any carrier, locked or unlocked, on eBay now, can't you? I don't see any reason to suspect that, even though the iPhone has a particularly onerous contract, people won't still buy one, get bored or otherwise dislike it, and decide to sell it to you, even before the first wave of post-contract iPhone 3G's becomes available in 2010.