I want to start living side-by-side with Sivlerlight 2 and 3. Its not that easy, but it can be done.
Up to now I've been leaving my SL3 development on a VPC, and that is clearly the safest route. If you aren't ready to maybe screw up your Visual Studio installations with Silverlight 3, don't follow anything I am going to tell you.
First, there is Visual Studio. I found this great set of batch files for switching back and forth:
Second, Blend 2 SP1 and Blend 3 will install side-by-side.
I haven't done this yet, so wish me luck (in the morning...its too late to try tonight). Buf if you've had luck with this, let me know in comments!