Well, there's a HUGE thread on the Apple.com support forums indicating that a whole bunch of users have lost the ability to sync or otherwise interface with their car's BT systems since the release of iOS 4.
Having said that, these BT issues (failure to sync with car, garbled audio on the callers end, pairing issues) are like the proximity sensor problem - There many with ZERO issues and there are also many that are so frustrated they want to chuck their phone through the front glass of an Apple store.