i've had varying success with drive-thrus...
In Texas (mostly Austin and Houston) about half the time they just looked at me really funny and served me and about half the time refused to serve me quoting 'corporate policy' and would require that i come inside (i often had no lock so i would bring it inside and sometimes got trouble with that too). A few times late at night when inside was closed i was simply refused service - one time as i was arguing that as a cyclist i had the same road rights as a car and should thus be able to use the drive-thru and should not have to go home and then get my car and drive 2 blocks to the Taco Bell to get served, some guy waiting behind me just said 'jump in' and i ordered from his car and then got back on my bike...
in Massachusetts i usually got served in the drive-thru, but i think only b/c bikes were so rare the workers weren't aware of 'corporate policy' and i of course never reminded them.
in bike-friendly Portland most of the drive-thrus quoted their 'corporate policy' of no bicycles in the drive-thru b/c of 'safety issues' which they were never too clear on.
i iniquired as to what this means and was told that cyclists or pedestrians were a robbery threat to the workers --- come on how is it easier to pull a gun on someone or threaten someone from a bicycle than from a motor vehicle? the only difference is that the car SHOULD have a license plate, but anyone robbing a drive-thru should be smart enough to cover or change the plate (uh, well, maybe not?)
anyway, with a particulat Taco Bell right by my house in Portland i was about 90% refused service except for when one guy was working and his manager wasn't there (he was a cyclist). one time late at night when only the drive-thru was open i was refused service and i had to ride home, get in my car, drive 4 blocks and was then served ---- but i find that utterly ridiculous, especially as i believe in using cars as little as possible in urban areas and their policy REQUIRES that i use my car, regardless of how convenient walking or cycling may be.
then i wrote to the Taco Bell Corporate office since i was told that was who made the rule. they sent back some generic form letter "thank you for your input and we value your business and your comments and will look into it..."
Does anyone know the real reason why the general policy is 'no bikes in the drive-thru'? i suspect that they are afraid of a liablity suit if some idiot car driver runs over a cyclist, but the risks are no greater than riding your bike on the road (less b/c speeds are so low)
as an aside, when i used to use my motorcycle i was NEVER refused service. if it's a safety issue, how are bicycles and motorcyles so different? (yes, motorcycles are registered and should have a valid plate)