Bypassing AV with metasploit is a hot topic and there are a few different methods to attack this. I think this article is one of the better methods for evading AV considering it gives the tester plenty of flexibility in obfuscating ASM instructions and allows for bypassing of Static Binary Analysis and Heuristic based AV engines.
http://www.pentestgeek.com/2012/01/25/using-metasm-to-avoid-antivirus-detection-ghost-writing-asm/
Have Fun!