When I recently picked up my 20D I also bought a 1GB Sandisk Extreme which looked suspiciously bulky in its package as though the contents had been removed and not replaced correctly. It was just a fleeting impression of little consequence. These disks are guaranteed for life.
When I downloaded the first shots using the Canon software (a remarkably troublesome procedure) the images came in 2 folders, 899 and 900. The first folder contained MG_9906.CR2 to 9999. The second folder MG_0001.CR2 to 0008 giving me the impression the camera had fired almost 10,000 shots before I got it and was on its second round.
I then formatted the card and was back to MG_0001. What's going on here, then? Is it likely the card had been used for almost 10,000 shots before I bought it, or did someone briefly test it in a camera that had almost 10,000 shots on the counter?
Slightly off topic, after formatting the card I was unable to download further images using the Canon software. Just couldn't figure it out. Deciding I wasn't going to waste any more time, I bought BreezeBrowser's Downloader Pro which works like a charm.