I haven't found a discussion regarding this yet, and I've searched for quite a bit.  I've come up with a solution that resolved the issue, but it's not exactly elegant.

After upgrading from the Betas to CTP, I've come across the following client side error, which never appeared before: (nstuff debugging info)
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 Microsoft JScript runtime error: Sys.ArgumentUndefinedException: Value cannot be undefined.
Parameter name: type 

at http://localhost/[AppName]/ScriptResource.axd?d=u_zg0lxZbCGICB2KkUULAzBlgWJw9WItXUSrW_r9mEP1jnmhPKIpcfnOBSKPZhBU4C2cNfYdPSyK4qOW1AirYrmZDRaJMV_UKr2pdXxNfCY1&t=633018166467500000 [2811]
    Sys$Component$create
    code: throw e
at http://localhost/[AppName]/ [294]
    JScript anonymous function
    code: $create(Sys.UI._UpdateProgress, {"associatedUpdatePanelId":null,"displayAfter":500,"dynamicLayout":true}, null, null, $get("ctl00_updateProgress2"))
at http://localhost/[AppName]/ScriptResource.axd?d=u_zg0lxZbCGICB2KkUULAzBlgWJw9WItXUSrW_r9mEP1jnmhPKIpcfnOBSKPZhBU4C2cNfYdPSyK4qOW1AirYrmZDRaJMV_UKr2pdXxNfCY1&t=633018166467500000 [2492]
    JScript anonymous function
    code: evt[i](source, args)
at http://localhost/[AppName]/ScriptResource.axd?d=u_zg0lxZbCGICB2KkUULAzBlgWJw9WItXUSrW_r9mEP1jnmhPKIpcfnOBSKPZhBU4C2cNfYdPSyK4qOW1AirYrmZDRaJMV_UKr2pdXxNfCY1&t=633018166467500000 [3854]
    Sys$_Application$_doInitialize
    code: handler(this, Sys.EventArgs.Empty)
at http://localhost/[AppName]/ScriptResource.axd?d=u_zg0lxZbCGICB2KkUULAzBlgWJw9WItXUSrW_r9mEP1jnmhPKIpcfnOBSKPZhBU4C2cNfYdPSyK4qOW1AirYrmZDRaJMV_UKr2pdXxNfCY1&t=633018166467500000 [49]
    JScript anonymous function
    code: return method.apply(instance, arguments)

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There were no code changes on my app since I upgraded to the CTP. 

 I isolated the problem to be the following - I have an UpdateProgress control in the master page.  On pages where there is an update panel in the body - the above Javascript exception is not encountered.  On pages without - the exception is hit.  I initially addressed this on a page by page basis to confirm this was the cause - simply placing an empty UpdatePanel, generically named and without any real purpose - and this solved the problem and the error was not hit.

 For simplicity, I just threw an UpdatePanel, again empty - as in the following - in the Master Page, which solved the problem across the board.

I'm not concerned here, since the solution is working.  However, I am puzzled by the cause and resolution - is there now a requirement to include UpdatePanel in order to utilize an UpdateProgress?  This may seem odd - in most cases one really is not useful without the other, however, placing my UpdateProgress in the Master page was simple enough - it's utilized when needed, and if not needed, it's not exactly a burden to have sitting around.  I just found it odd that I had to fudge in a useless control in order to avoid this error. 

I apologize if this post was a bit verbose and meandering. I spent a bit of time trying to figure this out, and after benefitting so much from all the other posts on this board, I figured I'd try to contribute a bit.