It doesn't take much bandwidth at all to do this. Even site to site VPN over DSL connections would be fine so long as you're not making massive changes to AD all the time, which most people don't. The only thing that would take awhile would be the initial seeding of the AD database, and you can even use a backup from an existing database to seed it upon promotion if you're concerned about eating up bandwidth.
Another way to keep bandwidth utilization low is use Windows 2008 domain controllers in your entire domain, raise the domain functional level to 2008, and transition to DFS-R replication, which is significantly more efficient than FRS. But honestly, unless you're using dial up modems to link the two sites, you shouldn't have a problem.