Here's what happened.
brand new sandisk 4gb ultra II SDHC. Cam is a rebel xs.
took about 300 pictures and then the camera sort of "locked up" on one, error message came up that it couldn't read the card. shut the camera off, and turned it back on.
when I turned it back on - there were no pics on the card - like it had been reformatted. really srtange - Thankfully I had copied the pics to the computer in the mean time - but it surprised me all the same. I've never heard of that happening before. looking for some information if anyone has had this happen to them. perhaps I just got a bad card.