The MSBuild team just posted beta 1 of the “MSBee” toolkit which enables building .NET Framework 1.1 applications with MSBuild. For those of you moving towards MSBuild and still have a few 1.1 apps lingering this bring the build process under a single tool.
On a related note I've written a few custom tasks already to generate publisher policy assemblies, use FxCop 1.35b1, Nullsoft Installation System, and the Red-gate SQL Toolkit. I'm working to get them added to the MSBuild Tasks Community Project.
Source: MSBuild Team Blog - MSBuild Extras - Toolkit for .NET 1.1 (beta 1) is Out!