According to Boy Genius Report, the iPhone 3G is very likely to be discontinued. Apple has reportedly stopped supplying AT&T stores with the 8 GB iPhone 3G. Additionally, the Australian online Apple Store lists the iPhone 3G as "currently unavailable," and UK carrier O2 has altered its ordering options in a way that makes it impossible to purchase the low-end iPhone.
It's not at all shocking that the iPhone 3G is most likely about to be discontinued. The two-year-old hardware is relatively anemic even compared to its successor, the 3GS. Because of its lower RAM and CPU speed, the iPhone 3G won't enjoy all the benefits of iPhone OS 4.0, particularly multitasking. Once the iPhone 3G is discontinued, one of two things will probably happen -- see them both after the link below.
1. The 16 GB iPhone 3GS will fill the low-end, US$99 spot currently held by the 8 GB iPhone 3G.
2. The iPhone 3GS may be discontinued as well, with the forthcoming iPhone filling all price points. Although so far, there's no indication that the 3GS is experiencing the same sort of shortages as the iPhone 3G, which makes this scenario far less likely.
Bottom line: if for some odd reason you still wanted to pick up an iPhone 3G, it may already be too late. At this stage, though, that's probably a good thing.