I'm honestly not 100% sure of the rules/legalities. I think a lot would depend on where you're employed and what the your corporate rules are combined with state guidelines.
I believe the Federal 2009 Reimbursement Rate is $0.55/mile.
Not every company reimbursed at the federal rate - in some instances you can try to claim the difference on your individual taxes.
If you are getting reimbursed the amount of reimbursement is taken care of so you can't claim it as an expense - you've already been satisfied by your employer.
Commuting is not business mileage.
I know there are states where an employer is required to reimburse.
I'm sure some states have no such requirement. I'd consult either a local tax consultant or maybe try the DOL for guidance or call the state tax reps for guidance.
As for no reimbursement if it exceed the cost of a plane ticket - I could see that. They would most likely need to have some kind of process in place.
For example someone making a round trip between phoenix & tucson you'd be looking at about 232 miles. That would be about $128. Could you find that for a roundtrip? - Maybe/Probably Not.
Now RT from Phoenix to Vegas would be about 580 miles or about $319. You could beat that with airfare most times.
Another corporate consideration is lost work time due to driving, insurance concerns with so much over the road time, etc where if there is a longer distance they may prefer you to fly.
