Remy - The 4.0.1 update should not change the amount of signal you truly receive. According to Apple, it only corrected the thresholds at which they display each bar. So, a 'bar' is not really a useful measurement. They could have shown a %age, but no one does that.
Also, here's an article about Bluetooth issues:
Bluetooth issues with iOS 4? | iPhone Atlas - CNET Reviews