From what I recall of this issue with the 2.1, it's not actually hanging up on the call, it is MUTING the call. Since I upgraded to 2.2, it happens whenever I shift the phone around and use the 'ol "shoulder hold". The caller is still on the line, but they can't hear me. I have found that hitting the "back" button puts me back in talking mode and all is well. It is definitely a problem with Froyo, because I never had this issue with 2.1. My phone isn't rooted, and I noticed it started up as soon as I upgraded.
There are some apps on the market that lock the screen during calls, but they are a pain to use regularly. I have my phone unlocked (so it doesn't require security after coming back active from standby), and I like it that way...adding one of those screen lock apps would irritate me almost as much as the muting issue, I think..lol