The validation key is a string of 20 to 64 random, cryptographically-strong bytes, represented as 40 to 128 hexadecimal characters. Longer is more secure, so a 128-character key is recommended for machines that support it.

Problem description:

After configured ASP.NET on all the servers, once enabled view state machine authentication code, client got a View State Error.

Solution:
By default, ASP.NET creates a random validation key and stores it in each server's Local Security Authority (LSA). In order to validate a ViewState field created on another server, the validationKey for both servers must be set to the same value. If you secure ViewState by any of the means listed above for an application running in a Web Farm configuration, you will need to provide a single, shared validation key for all of the servers.

The above information is excerpt from Susan Warren's article on Microsoft® web site. The whole article can be found here:
http://msdn.microsoft.com/library/?url=/library/en-us/dnaspnet/html/asp11222001.asp