I don't recall the exact numbers, but I think I had only used about 4gb of the 8 available in my old iPhone. I had 2 full length, iPhone formatted movies; about 400 pictures, maybe 100 songs, and about 10 video podcasts. I would regularly swap the podcasts out for new ones, as well as the movies, so I never had more than that. As far as music is concerned, my 100 songs only took up about 400mb, so even if I added several hundred more, it wouldn't have become problematic.
Now, I am fairly experienced in the smartphone world, and I have never seen more than a small handful of apps that have an install footprint larger than a few megabytes. That said, I think you will be hard pressed to really fill up the space unless you go app crazy. Games may take up a bit more space, but if a full PSP (some, not all) game takes up 500mb on an SD card, I can't see iPhone games being much larger.
If you are a music and movie nut, and insist on having a several thousand song library at all times, along with 4-5 full movies, it may be a different story... I just don't use that much space. For an example, I had a 40gb hard drive in my computer until I upgraded to my iMac in 2006, and I never even filled that. Now in my iMac, I have a 250gb hard drive with around 230ish available from the word go, and I still have 200 free gb...
Anyway, my guess is that they release a 16gb and a 32gb version. I will personally go for the 16 because of what I said above.