I have read with apprehension the reports of memory card errors on the Canon S2 IS, and it seems that many of the problems have arisen with 1Gb or 2Gb memory cards. My only personal experience is with a 512Mb card purchased from "Crucial Memory" in the UK. To date this has performed without any errors over a period of 4 months. Whenever all photographs have been downloaded to my PC I always perform a low level format on the card I have used "Crucial Memory" in several PC's that I have built over the years with 100% reliability, and their website is extremely useful and detailed.
I would be interested to hear if anyone else has experience of "Crucial" memory in their Canon S2 IS, as I am considering purchase of a 1Gb card to give me some movie capability. I personally rate the camera very highly as a (very) amateur photographer, although it takes a lot of reading and experimentation to become familiar with all the excellent features of this camera.