EXCELLENT post! As you said, all things considered, converting a 120V circuit to a 240V circuit is pretty simple when you know what you are doing.
Tokudai - Now I really suggest that you get an electrician because what you wrote above is not what I said. No offense intended, but you seem to lack even the most basic understanding of electricity and circuitry - either that or you just have difficulty expressing yourself.

Originally you were talking about installing some new wires to accomplish a certain task, now you are talking about CONVERTING something??? This makes no sense whatsoever in the context of your original post and, anyway, "converting" things is how unknowledgeable people really fuck-up their wiring. If you add a new circuit for your project, about the worst thing that can happen is that it won't work or it blows the circuit breaker. However, if you go messing with existing circuitry, you're liable to cause a lot of damage to your home and everything electrical/electronic in it.

Get an electrician! You don't need a licensed contractor, any old wire puller can do this.