<p>In VS2005 and up, is it possible to specify which configuration should be selected by default?</p>

<p>I have several configurations in my solution but one of them should be used in most cases. Hence I'd like to make sure that devs who pull it out of Source Control use the right configuration(unless of course they specifically choose another one).</p>

<p>Ideally, this setting should be in the .sln file since that one is under Source Control.</p>

I don't know if I understand the complete situation behind your question, but there is a way to "externalize" some of VS.Net's project configuration values such as FxCop settings into a separate file. Then you could have all projects in a solution point to that file. However, I don't think it would allow for your devs to chose their own configuration after doing the "externalization"
If this seems like the right path for you, check out these posts from me in regards to coding standards on how to make similar changes for configuration.
My Seattle Code Camp Presentation on Coding Standards and Code Review
Automating Standards and Best Practices