This is something I heard some of the guys at the indoor training ride talking about. Yup, sometimes they interfere. Sometimes they interfere with each other - the guy next to me moved further away /cause his monitor was showing my heartbeat... well, at least that's what he SAID, I *had* taken a shower!
If I sit at my laptop with the HRM, it'll go berserk too... right now it's at 214. Funny, it never got past 170 (that I saw) on the indoor race!