can I use my OS X Lion upgrade on a different machine?
It is tied to the new mac. The store verifies the mac rather than the Apple ID to allow the download if you need to reinstall the OS.

Some people are reporting that the few macs that shipped with Snow Leopard from the factory, but were changed later get a redemption code that works more like you purchasing an upgrade. That purchase seems to work on 5 macs like the other App Store purchases.