You may at some point come across a hanging apple with spokes that never turns into OSX. This happened to me. This occurred after a recent Boot Camp install. It was specifically after I installed IE 8 update to the windows partition (not sure if this was a cause or coincidence).
After researching this problem a little bit, it seems that there is a widely held opinion that this problem cannot be fixed. For me the solution was:
1. At the startup chime press and hold command-v. This enables verbose mode and will allow you to see where your mac is hanging. Feel free to google whatever comes up.
2. Restart in single user mode (hold command-s). This enables single user mode.
3. At the prompt type
4. Repeat the above command until it tells you the volume is ok.
5. Cross fingers and restart.