如果你在WebFarm上使用需要加密的ViewState，则必须要让WebFarm上所有机器的MachineKey一致，否则ViewState将会失效，这篇文件就是讲述这个问题的，希望使用WebFarm技术的哥们可以看看。
The <machineKey> Element configures keys to use for encryption and decryption of forms authentication cookie data and viewstate data, and for verification of out-of-process session state identification. This section can be declared at the machine, site, and application levels, but not at the subdirectory level. For anybody that's running ASP.NET on a server farm, the <machineKey> element is one of those guys you want to know as much as you can about IN ADVANCE - before you run into problems!
http://www.eggheadcafe.com/articles/20030514.asp