I too have a canon S2 and the same memory problem. with a 1GB Kingston Elite Pro, the card sometimes gives a "memory card error"
. The pictures can no longer be viewed on the camera. However, it is possible to put the sdcard into a USB memory card reader and download the photos into the computer.
Once that is done I can reformat the card on the computer (using FAT not FAT32), then put it back in the camera and do a low-level format there. Everything works fine till the next 'memory card error". It is annoying.
What is interesting is that I also have a 256M Kingston Elite Pro, as well as a regular speed 256M Kingston SD card. neither have given any problem.
So I suspect the problem is with the 1GB card specifically. If you look at the other posts on the web, a lot of complaints are with the 1GB card, but i hardly find any for lower capacity ones.
ps: it may have something to do with the battery/power requirements. On a couple of these occasions, the problem occured when my battery was running a bit low.