So you own an Odyssey and would like to have the machine accept the new twenty dollar bills (as of late 2003)? Here's the short answer - it's not gonna happen.
Why? The bill acceptor used, a Mars Electronics ZT1101US, was a custom validator designed specifically for the Odyssey. Voltages, pulse lengths and timing were all unique to that model. Since the 1100 series is no longer produced by Mars, updates are not available for the new twenty dollar bills or any other upcoming new currencies.
Mars has introduced a new line of acceptors that are similar in overall design - the 1200 series. The problem is, to implement one of these into an Odyssey would require system software modifications. With the buyout of Silicon Gaming by IGT, all software development for the machine was halted. In short - no new games, no updates, nada. For all intents and purposes from IGT's standpoint, the Odyssey is a dead platform.
This information was supplied directly by Mars Electronics technical department and can be deemed reliable.