Once you've loaded an iPhone with media, 16 GB can feel a little cramped. There's no price drop on the 32 GB iPhone 3GS, so if you need more space, it's best to wait for Apple to announce its next generation of phones.
Have you ditched your digicam? Would you like to?
The iPhone 4G's rear-facing camera will probably pack more megapixels, but the most important addition is flash. That'll come in handy for people who'd rather leave their digital cameras at home for uneventful nights out, but still want to capture the unexpected. If you don't dare go anywhere without a compact digicam, the upgraded iPhone camera's no biggie.
Are you a videophile?
Gizmodo wasn't able to determine the resolution in the iPhone prototype , but suggested a 960-by-640 resolution display -- twice the pixel count of the iPhone 3GS. There's nothing wrong with the iPhone 3GS display, it'll just be inferior to whatever comes next.