Dan Moseley, dev lead for msbuild, wrote a debugger for msbuild while on leave(!) back during the 2010 cycle. While it ultimately couldn’t be fully completed and tested to be an official feature, it is in the product with a switch to enable it (it’s off by default). This was a conscious decision made by the VS team, and I think it was a great compromise. It wasn’t something feasible as a power tool, and it’s too useful to have to wait for the next release.
Dan has written a series of blog posts on how to use it.