On the other hand, you might be looking at an iPhone 3G right now because of its very low price (US$99 with a 2-year AT&T contract; some international carriers are even offering the iPhone 3G for free with certain plans). In the case of the iPhone 3G my buying advice is slightly different, but I can still condense it down to one word: DON'T.
The iPhone 3G is nearly two years old now. That's enough reason by itself not to drop money on this model of iPhone, but thanks to the upcoming iPhone OS 4.0, there's an even more compelling reason not to buy it: Apple's next iPhone OS won't offer all of its features on the iPhone 3G. Specifically, the iPhone 3G will not be able to take advantage of multitasking, the ability to run several third-party apps at once.
To some of you potential buyers that may not sound like a big deal, but there's one more thing to consider: the original iPhone is only a year older than the iPhone 3G, but support for the original iPhone is being abandoned entirely as of OS 4.0. There's no way to know yet if dropping support for iPhone models after three years on the market will be a trend, but the safe bet says yes. This means that if you buy an iPhone 3G today, you might only have a year of officially supported software updates from Apple before the company considers your iPhone too obsolete to run its latest version of the iPhone OS. That makes the iPhone 3G an extremely unattractive prospect right now, no matter what the price.
It seems odd that Apple is even still selling the iPhone 3G at all, considering that it won't enjoy full support for the next OS update. Apple stopped selling PowerPC Macs years before making an Intel-only OS with OS X Snow Leopard, so while G5 Mac owners were dismayed their Macs had been rendered obsolescent so early on, at least they had a couple years to get used to the idea that their Macs might not be supported forever. The current situation with the iPhone 3G is a bit like if Apple had still been selling G5 Macs only a couple months before Snow Leopard's release as an Intel-only OS, and some people are furious about Apple's lack of full support for the iPhone 3G (not-safe-for-work language at that link).
Once the next-generation iPhone is introduced (most likely in June), it's likely Apple will discontinue the iPhone 3G altogether. Personally, I think Apple could have avoided a lot of ill-will toward the company by discontinuing sales of the iPhone 3G upon the iPhone OS 4.0 announcement; Apple's going to have a tough job explaining to customers why their "new" iPhones aren't fully supported.
The bottom line: if you have yet to jump on the iPhone bandwagon, don't get an iPhone 3G.