A friend of my had this exact same thing before. I don't remember the exact cause, but I remember the fix.
You have some extension or other that's causing the hang at startup.
Try enabling/disabling different combinations of extensions and rebooting and you should be able to narrow down the culprit.
To make life a little easier, get a copy of Casady & Greene's Conflict Catcher
which totally automates the process as well as providing several other useful features over Apple's Extensions Manager.