Yes, I see this too (on the Vibrant - Tmobile version). I think it is because the messaging/contacts app is trying to pull contact info from multiple places and not recognizing when contacts are the same person. Merging data like this is quite difficult and I'm not not surprised the software is getting it wrong. Also, it seems to be pulling email addresses from my "sent" folder and adding them to my contacts list. I don't like this at all - it needs to be an option that is off by default.
I remember turning off some of the accounts that were included but I can't find where that option is now.
I think it was on the main screen -> menu -> settings -> Accounts and Sync. I disabled everything except gmail.