There's nothing built-in yet, but we do have a feature tracking it that I'd encourage you to vote for: http://pytools.codeplex.com/workitem/691

The priority of this has gone up for us after PEP 405 but we weren't able to get to it for 1.5. You can go to Tools->Options->Python Tools->Interpreter Options and setup a custom interpreter which points to your virtualenv, but we need to make it easier than doing that.