So thanks for all the comments and help, guys, but I think (and hope) the mystery has finally been solved. Last night, one of my roommates came home and decided to unwind with a game of Guitar Hero II. I wasn't paying much attention, but the conversation at some point shifted to my most recent memory card death, which I hadn't had a chance to tell him yet. I was just starting to tell him that I thought it might be Carl's (my other roommate's) PS2 that was causing the problem. Just as I got that sentence out of my mouth, he attempted to load his game data, and it came back corrupted. I jumped up and yelled with glee, "Yes! Yes! I knew it! It is the PS2!" Then I realized what I had just said, sat down and said, "Boy... I'm really sorry. That sucks." But it was just so nice to finally see an answer.

Denoument: Carl doesn't seem to see the reason why he should pay my roommate back for the memory card it ate, but he agreed to anyway. Since C3PX was kind enough to send me his old memory card for free, I decided not to try to get any money out of Carl. However, I'm still convinced the first memory card I got off eBay was a dud simply because it didn't work from the moment I put it in the PS2.