The reason is that the conventional wisdom suggests that Apple will be announcing a new 3G iPhone within about a month or so. Thus, many offer the advice of wait until the new iPhone is announced. Now, you do have to factor in that it might be several months after it is announced before it is actually available. This is partly due to how Apple has worked in the past with iPhone announcements...but can also be due to FCC approval (it takes a little while for final FCC approvals before a wireless phone can be sold...so if Apple has not started that process, it coudl be while be for they can sell it).

If you like the current iPhone and are not "drooling" over the idea of a faster data connection (i.e. the 3G connection), then it might be worth going for it now...but then you might just save some money after the announcement (i.e. they could drop the price when the new iPhone is annouced or released). Thus, it is really a question of can you wait or not...if you have a pressing need for a new phone and you want it to be an iPhone, then you might just go for it. If you can wait, however, you might be better served by waiting...to either get a better price on the old version (assuming that they will still sell/carry it) or get the new version when it comes out.