Mark Miller has posted his code for a ViewStatePersister using the "common sense but not obvious" GUID technique that was outlined previously by Scott Mitchell and myself.

He stores a GUID in the ViewState hidden field, and sticks the bloated ViewState in a temp file on the server.  It doesn't solve the problem when running multiple web servers while using stateless balancing (meaning: NOT using sticky sessions/node affinity) but it's the most elegant and complete solution I've seen yet and should work great on a single web server. 

A few questions I have though:

• When do the files get cleaned up and how often? Do you clean up old ones in a background thread within ASP.NET or a separate Windows Service?  Thought: I wonder if you could delete them after immediately after the Load?  You wouldn't be able to RE-post data, but it'd be cleaner, no?
• GUID generation is very expensive, and can really slow you down under load.  I wonder if it would be faster/easier to have a single long and use InterlockedIncrement or  InterlockedIncrement64 to safely increase the value on each call until it overflows and you start again at 0.

Many thanks to Mark for sharing!