I have been doing this sort of thing now for over a year. The mic is the hardest part...if you do move around a fair amount, you do need a wireless mic. Forget bluetooth, I was hoping for the same thing. The head-set option is one (for mobile phones), but the range is not very good, and the sound atrocious. A very good audio-quality for a relatively low price is this solution
NADY wireless lapel
http://www.dak.com/reviews/2030story.cfm?Ref=G&PM=WM&type=GSrch&Srh=wireless_mic rophone
This is the next level (much better value)
Samson Stage 5 Lavalier
http://www.americanmusical.com/item--i-SAM-SW05SLM5--m-0254933.html
Then it gets really expensive, so I spare you that...
The built-in mic is not that bad, actually - you just need to be not too far from the Mac and crank it all up. The down-side is ALL the background noise gets picked up big time, which doesn't make for good listening over headphones.
So the lavalier is your best shot.
Be absolutely sure to check out (and I strongly recommend using) ProfCast
http://www.profcast.com . This has saved me hours of work and does a wonderful job at producing a real "lecture experience" that students really appreciate. The lecture together with the slides exactly the way they see them in class! In the latest version you can even take screen shots when you browse the web or show other things. It's simply amazing (and no, unfortunately I am not affiliated with them or get some kickbacks 😉
Hope that helps!
MacBook Pro Mac OS X (10.4.7) AKG wireless
MacBook Pro Mac OS X (10.4.7) AKG wireless