Tim Heuer has instructions on how to setup a multi-targeting SL 2/3 configuration using VS.NET 2010: Silverlight and Visual Studio 2010 Beta 1

Giving it a try right now and will report back if I have any issues with his instructions.

Update:
Well, I've tried this, and it seems to work.  Specifically, what I did do was install as described, and I used 2010 to open an SL2 project which it did fine, and I could also create new SL3 projects, which could not be done with SL2 and 2008 as you had to use either SL2 or SL3, without any possibility of both. 

I didn't really try to change the target, as the scenario I mention is my use case that I need this for.

However, I'm not sure I'm going to be using 2010 too much in this beta as I just was annoyed by various issues.  A few times starting the debugger it locked up, and I can't seem to change the default colors in the editor to be what I want.  Also, coderush / refactor pro don't work in it either.