On my third or fourth, depending on how you look at it
The first one, I got on launch day, and it worked perfectly for a year and a half (including many hours of intense Gears MP). Got the RROD, MS didn't have their repair service fully up yet, so I took it to the EB I bought it from, and after some haggling got a free replacement. Took it home. Got the RROD immediately. Returned it AGAIN the next day (they were quite skeptical that it wasn't me who caused it). Got them to check if the next replacement was working while I watched them. It was. Took that one home, and it was fine for a while.
That is, until I downloaded the Halo 3 Beta.
RRoD about 20 seconds into my first match. Not happy. Sent it into MS, and got it back in two weeks, leaving only one week left for the beta :S
So, yeah, that's the one I'm on now... either my third or fourth, depending on if you count the one I had to take back immediately.... it's the ONLY problem I have with MS with regard to their gaming division.