Ok.. so I have 3 xbox 360's in my house. Why? Because I have 2 teenage boys, and we love COD4. Plus we use the 360's as mce extenders...

Anyways: If I buy the maps, do we have to pay for them 3 times for each xbox? Is there a way to buy them once and re-download them again on another xbox 360?

Well, if you have a big enough memory unit (I don't know how big the map file is), say the 512MB for each machine, you might, MIGHT (I'm not 100% sure) be able to copy the file to each memory unit. If I'm wrong, I'm sure sure someone will correct me pretty quickly. I have no way to test this as my 360 is in for repairs

I'm pretty sure its one map pack per xbox live account.

I found this to be untrue by accident just yesterday.. I myself have 3 xbox 360's and paid for the map pack legit on all 3 consoles.. But yesterday i decided to make a new account on the 30 day trial to test the new god mode mod and i spg'd my new account with my original account to get all the perks after level 4 and low and behold to my surprise the map pack worked on my new account.

Oh and btw my new account is stored on a 512 mb memory card with no mappack stored on it.