That's an interesting idea and I problem I never realised existed before (Australia uses 240V so there are no problems using European equipment). A lot of equipment these days can auto-switch between common supply voltages - I'll check if the 12V transformers can do this as it negates the rest of this post, but due to their age I doubt this is the case.
The way I see it there are two ways you could solve this - use a step-up transformer to convert the 110V to 220V to supply to the Lego transformer. Or if you know what you're doing (ie. are a handy type of person and have a good knowledge of electricity and it's dangers) you could open the transformer up and replace internal transformer with one suitable for plugging into a US outlet.
Unless you get confirmation that it will work, don't go trying to use a travel adaptor to plug a 12V transformer straight into 110V mains - you might release all the smoke they packed in!
Edit: you can negate all of this messing around if you're happy to go with non-Lego parts. Just buy a 12V 1 amp wall wart and you're off and running...
Edited by Beej, 16 September 2009 - 06:26 AM.