Don't buy it! The most important reason is because you will not be eligible for a subsidized upgrade when the next model is released next July. Waiting has many benefits and very few drawbacks. You will save $15 per month on not having a 3G data plan w/ texting AND you will not lose subsidizing pricing, so you will save a lot of money. If your plan is less than $99.99 per month for a single phone line, AT&T's rules are that you have to wait at least 18 months before they will give you discounted pricing on a new phone. Secondly, 3G doesn't add a whole lot of functionality. 3G speeds are faster, of course, but anytime you can use WiFi completely negates that fact. The GPS isn't very useful yet either seeing as there are no turn by turn directions, etc. Of course I'm sure this will come along before next July, but how many months will it be until it does come? Consider how useful GPS is to you as well.
The best reason to hold off is the next iPhone model is
very likely to have important hardware upgrades that the original and the 3G will
never have, like:
Better camera - that's just a no-brainer.
More RAM, better processor - technically the 3G processor is overlocked (i.e. "sandbagging") but what it boils down to is the next model will have better hardware to run more intensive apps.
More flash memory - obviously, but this is never a bad thing.
Stereo AD2P bluetooth - listen to music with bluetooth headphones with stereo sound, not just for phone conversations!
More external buttons? - an additional physical button can add a LOT of functionality. For example, it could be used as a shutter for the camera, to activate voice dialing, or maybe even programmed to run whatever app you want. Whether or not the new model does have something like this, it is certain that the 3G never will
Video recording and MMS! - Yeah, they can be added by software, but it has yet to happen, and I really doubt these features will come to the current iPhones before next summer. And how fun would it be to have a brand new phone when we get these features
What else can you think of?
I'm not saying to wait forever for better features and never buy a new one. But in my opinion, the 3G doesn't add enough to justify the money. You save on your monthly bill, you don't have to shell out $200 right now, AND you keep your subsidy so you can buy the new model next July for cheaper. If you upgrade now, then decide you like the new model enough to buy it then, you will have to pay out the full price to get it. Just consider all things for yourself and see if you think it is worth it.
P.S. I voted "I love you."