You hit just one of the typical NuGet/xbuild compatibility issues, including the one I reported a while ago,
http://nuget.codeplex.com/workitem/4051
The workaround you find like using the latest NuGet.targets can help, but ultimately Microsoft guys have implemented a brand new restore approach (which has been deployed already to all Microsoft platforms), now it is time to wait for Xamarin to catch up.
(Updated: Now you should use
mono --runtime=v4.0.30319 .nuget/NuGet.exe update /self
mono --runtime=v4.0.30319 .nuget/NuGet.exe restore my.sln
to restore packages.
More information can be found at NuGet site)