NuGet package restore does work on Mono with xbuild, but you may find some *nix builds fail because the imported target that uses $(SolutionDir) fails on Mono due to a double forward-slash in the path. As xbuild doesn't support property functions I found this crued solution ensures Visual Studio doesn't re-add the import line but does allow xbuild on *nix to complete.

<Import Condition="$(MSBuildProgramFiles32) == ''" Project="$(SolutionDir)\.nuget\nuget.targets" />
<Import Condition="$(MSBuildProgramFiles32) != ''" Project="$(SolutionDir)\.nuget\nuget.targets" />

If you've found a better solution let me know :-)