Here's an odd problem....

 My UpdatePanel needed to work from a button click outside the panel, so I made the button update a hidden field in the panel that would trigger the refresh of the UpdatePanel (registering the button itself as a trigger wasn't working).

All this was working well and good until I decided to move the ViewState off the client and use the SessionPageStatePersister to keep the ViewState on the server.  The UpdatePanel still refreshes, but the hidden field _ValueChanged event is not firing anymore when the request is processed by the server.

It sounds like quite a hack to do it the way you are describing.  You should really focus on getting the button to act as an asynchronous postback trigger for the update panel.  Can you post the source code as it was when you were trying that?

Okham's Razor -  the simplest answer is usually the best.

Wouldn't help, DisturbedBuddha.  The reason the button wouldn't work is because it existed in a different ContentPlaceHolder in a master page.  So, while being a little bit of a hack, yes, it was the best solution at the time.  I'm actually assigning the button functionality when the page loads, depending on the context it's in.