URLRewritter & .NET 3.5 SP1

In one of our project, we encountered this error when we pushed our code to the server:

Validation of viewstate MAC failed. If this application is hosted by a Web Farm or cluster, ensure that configuration specifies the same validationKey and validation algorithm. AutoGenerate cannot be used in a cluster.

Our web server is a stand alone server, not a cluster/web-farm. So after digging around, there are multiple issues that related to our problem that can ultimately cause this error to show up:
1. From here: WebForm starts paying attention to the "action" attribute of the form tag in .NET 3.5 SP 1. So, if they are unmatched, this error will be generated.
2. We use URLRewritter, in which means that our URL in the location bar is different (modified) than the one in the action attribute.
3. In IIS7 or VS 2008 emulator, this problem cannot be replicated

There are also other issues that can cause this error (but they are not related to my project - and you can read about them here and here.

So, how did I solve my problem? I can just simply upgrade my web server to IIS7, but that was not an option when I was working on this. Here are the things that I did:
1. Add event handler to blank out the form action from global.asax.vb file for each request (read here)
2. Change form tag into the one supplied by URL Rewritter (read here)

Those 2 steps provide me with a URL that is identical in my action attribute on the form as well a consistent URL - which mean ViewState is valid!