All the documentation says there are no issues running them side by side, and it's been my experience that it hasn't caused any problems for me. I'm running VS 2005 Pro and VS 2008 Pro concurrently, though I confess I haven't played with 2008 much yet. One warning, however, would be against opening 2005 projects in 2008. It will automatically up-convert them for you, but if you need to open them in 2005 again (or a colleague who doesn't have 2008 yet), it can't be converted back to 2005.