As pointed out, Forwarding to Liechtenstein mobile on voicesticks free plan(next2Nothing) is $0.36. Forwarding to any of the IOM sims will cost you less than half of that, but finding an IOM sim that will provide free incoming calls (if it even works there) in the Bahamas is another story.

I am still surprised UM can provide service there at that rate. But, just in case it does not work, I am assuming you have a US sim (Cingular or Tmobile) that you can just roam on. Which mifgt be the best option anyway, since you will just be there for a week, and Bahamas prepaids have a very high initial cost.

And some IoM cards (Geosim at least) offer free forwarding any number in the world, including UM. However, adding another forwarding may make the whole process of setting up the connection much too long for the calling party :-P

The IOM sim freeGlobalSim also offers free forwarding to any number. Every number I have tested and used it with works 100% except for UM. With UM it was hit or miss. The forwarding actually went through and rang on the UM sim less than 50% of the time. Most of the time there was a message in a language I believe is german or so. Probably informing me that the call did not go through. Because of that, I have eliminated the idea of forwarding my IOM sim to UM in countries where imcoming calls are free on UM, and not free on the IOM sim.

I hate to be the dumb American, but...
I tried to purchase a United Mobile SIM and during checkout I am asked for an ID Type and ID Number (as required information)
What type of ID are they referring to?

It's a document that confirms your identity - passport, driving licence or national ID card. AFAIK this info is required by the Swiss law. However, I suppose they don't verify these data.