Actually, a wye-delta transformer can be used to 'derive' a neutral. So in the case of a reverse connected transformer (wye primary, delta secondary) the line-neutral voltage on the primary will be nice and stable and could in theory drive a load.
If there is any unbalanced loading that 'pushes' this derived neutral toward one of the supply legs, you will get circulating currents on the delta side that will tend to rebalance things.
The problem with using this derived neutral is (as iwire notes) that if you want to legally run 120V loads, you need to bond the neutral. Even though this is a derived neutral, you have essentially set up multiple neutral bonds, and you can get current flow on your bonding jumpers. This is also why you do not connect X0 to the supply neutral when you reverse connect a transformer.
If you could legally treat this derived neutral as a hot conductor, and not bond it, then you would be able to run 120V loads with it.
The OP asked the same question in a different thread, and the above can be added to my answer here
http://forums.mikeholt.com/showthread.php?t=126089#8
If the OP can get any delta to 480/277V transformer, in addition to the transformer that they are planning on reverse connecting, it could be used to derive a neutral (in the fashion of the zig-zag transformer mentioned in my other post.) But without knowing the size of the transformer and the size of the neutral loading, we can't be particularly helpful. Heck, the guy might be in Baltimore and only need 30KVA, in which case he should PM me
-Jon