This is a very important issue. Vesting and commitment should be decided upfront and in consensus.

While I could tell you my experiences with founders, I would encourage you to first read this two posts on vesting:

http://www.feld.com/archives/2005/05/term-sheet-vesting.html

http://www.naffziger.net/blog/2007/04/05/startup-stock-options-vesting-schedules-acceleration/

If you want help or advice on how to deal with it please give me a call. I would be happy to share my experiences around this subject.