Sorry to say, but the answer is no. There’s no option for expandable memory for iPhone right now. In order to expand the memory of a mobile phone, a manufacturer generally needs to include a slot for removable storage media like an SD card or MemoryStick. The iPhone has no slot for removable storage.
In theory, it’s possible that a third-party company could create some kind of expandable memory for iPhone by making something that attaches to the dock connector. As far as I know, though, there’s no such product right now.
So, while the iPhone 3G offers a good amount of storage, those of you looking for expandable memory for iPhone will need to be content with what you have now.