Since non of the .Netters chimed in I may ask the question up on the MSDN forum and I'll post back any useful information I get from there.

except that the darned asp.net forums aren't accepting my Windows Live Id as valid so I ain't going to mess further with trying to get logged in, three times is too many...

>Yeah, not mine either. :-) But an irritating bug it is. I was having data issues as a result of this and scratched my head until I ran it through debugging and watched some of the data changing. I gave up on a "solution" and just noted it so I wouldn't trust the control values on the fly.
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>It would be interesting to know what is happening internally here but I don't know how or have the time to deal with it. My guess is that ASP.NET does not update internal flags until later in the life cycle of the page, but that's just a guess.