I use a 16Gb card in my Lumix G1, so I never think of it in terms of a finite number of images. Even shooting RW2 for a week-long vacation, I've not come close to filling the capacity of the card. I'm more thinking along the lines of the post-processing regimen required, once back home.
So for me, my image files are structured in terms of folders on my computer. A naming convention that's date-relevant, as in "2010.08.04," for all the images off-loaded from the G1's memory card today, become the equivalent of a "roll of film." The main difference is that the quantity of images contained within this "roll" varies considerably, from a mere dozen or so taken on a short jaunt, to several hundred while on holiday.
Interestingly, my grandson is learning the photography ropes using my old Minolta X-370, so he's learning all about having only a limited number of frames with which to shoot, and learning the discipline required to anticipate when and where one might need to save a few frames for a special moment.
~Joe