Two new MSBuild community tools to check out!
Partho pointed me at a debugger he's created. It's on CodePlex too, which means you can help improve it. It runs as a logger, taking advantage of the rich event information that loggers receive, and the fact that loggers currently run synchronously with the build. (This will likely change in future versions of MSBuild.) Debuggers built on the logging infrastructure are about as good as you can get until we have debugger support built into MSBuild - but for many people that can be a lot better than no debugger.