Nothing I read talks about limitations of any kind, because it's based on an interface that like IMing except the IM is sent via Battle.net. And yet, as Shoju said, I seem to remember that it is limited by continental servers. I'll just PM Jane my email and we'll test this out for y'all.
Edit here:
Jane and I will continue continue as pen pals. RealID across continental servers did not work.
sniff