So I and some other mobile developers have been discussing where to host a new .NET mobile project: SourceForge or CodePlex?

SourceForge is the defacto choice. It's known, has a bigger community, offers Subversion, and is ahead in general features.

CodePlex is the Microsoft-specific answer to SourceForge. It's an upstart, uses Team Foundation Server as source control (which BTW you don't need Team System to use -- just use the free Team Explorer), but has the M$ money behind it and might be a better place to put a project if you want to get attention from the .NET community. Since it's smaller, you also might have a better chance to get noticed.

Unfortunately, CodePlex seems to be down this morning ... every page is giving me "The server has experienced an error."

Edit: it's back up now ... must have been a momentary thing.