When to raise domain functional level?

We very recently completed a project to retire two old domain controllers running Server 2003 R2. They are now replaced with shiny new 2008 R2 boxes. However, the functional level of the domain has not yet been updated for the 2008 R2 servers, just in the long-shot case of the need for a rollback to the old controllers. I expect to have the all clear to update the domain by next weekend.

I also want to note that our desktop clients are still 95% Windows XP. However, we're about to start a project to update our 200 or so clients to Windows 7 before the end of the calendar year.

Is there any advantage to holding the domain at the 2003 functional level while we are still supporting more Windows XP than Windows 7, especially given that some of the management stations are still XP?

Update: I forgot to mention earlier that we still have a pair of windows 2000 servers (not domain controllers) that support some legacy software. I'm working to replace those, but in the meantime I need to be sure that Windows 2000 can still participate in a 2008 R2 domain.