If you get the error

Validation of viewstate MAC failed / The state information is invalid for this page and might be corrupted

Then there are two possible fixes for the problem

1. <pages validateRequest="false" enableEventValidation="false" viewStateEncryptionMode ="Never" /> in web.config

- however this can open up a minor security hole as the request is not validated against the server

2. Make sure that the <machinekey> is the same for all of the machines in the cluster see here

There is more information about this problem in the following thread http://forums.asp.net/p/955145/1173230.aspx

As ever thanks guys for your help with this.