It's been suggested before, but so far Blizzard has only said it isn't currently feasible and left it at that.
However, Tigole recently posted the following:
Tigole wrote:
Yes, we have definitely thought about Cross Server Battlegrounds. It's a cool idea for several reasons and also solves some problems.Unfortunately, at this time, our server architecture does not support the concept. That doesn't mean that there isn't the chance that someday we could restructure things to accommodate the idea. It's a cool idea and we'll keep discussing it.
So while they can't do it quite yet, they are at least considering it. Cool!