OP, please don't take this the wrong way. But, I can never understand why so many people - you are not the first by any means - want to find a quicker way so they can miss the busses or sleep an extra minutes. I just want to explain why you might want to take the bus.
First of all it is dangerous to walk and you don't get to drop a bag at bag check. But heck that is up to you.
First the busses. They are part of the experience. Getting on a bus at 4 am, everyone nervous but everyone having the same goal. Seeing bus after bus pull in dropping off thousands of like minded runners. Walking from the dark "into the light" (like the person directing traffic always say). It is so cool, not sure I'd want to miss it.
As for the extra 30 minutes of sleep... well you probably won't sleep that well anyway. By this time you have put many many hours into running, probably many in the early morning. Don't sleep away the experience. A lot of people complain about having to get here so early. Heck I look at it as I GET to be there so early (except 2010). It is all part of the experience.
Go to the race, take the bus and get there early. You paid for the experience, don't cut it short.
enjoy,
Duane