Bit concerned that an old post is being linked from this URL http://msdnprod/library/en-us/dnaspp/html/viewstate.asp?frame=true - especially as

1. The URL of the person doing the linking is a Microsoft one
2. That post REALLY needs updating (and is just such a simple solution)
3. The post isn't even accurate, in it I state that you lose encryption (even if it is the stinky 3DES) using this method - umm...wrong actually you don't just that if you use compression the technique becomes rather ineffectual (since compression and encryption are kind of exclusive - you can't really compress random data...)

Anyway, I may just update that article with a rather more flexible solution - it is an oddly popular post!

Oh, incidentally that post presents a technique to compress ViewState - specifically it uses BZip2 to compress a DataSet Viewstate - BZip2 uses something called the Burrows-Wheeler Transform which makes it one of the most efficient general purpose compression algorithms for XML (as in compactness, it is a bit slow) . Intertesting algorithm actually, it was written by a guy called Julian Seward about 8 years ago and is one of the least encumbered by patents etc...

UPDATE: Should mention, I figured out what this was - probably someone in Microsoft reviewing and checking links on this article - this is just a stunningly good piece on ASP.NET ViewState.