I am getting this same error really quite frequently - in particular (but not exclusively) with:
PM> Install-Package SQLCE.EntityFramework
The work-arounds mentioned above do not always work.
An additional work-around I attempted is to copy the 'packages' folder (found in in the solution root folder of a previous solution)
To clarify - where the above command has previously succeeded in downloading the package (within some other solution) I find that I can copy the packages folder and the command then works (presumably because it no longer needs to do the download but, only
the install bit).
However - even the above workaround does not always work (it still did not work for 'Install-Package MvcScaffold' until I restarted VS2010).
This is quite frustrating and I hope it can be resolved before RTM.