I've tried to repro this (migrating from VS2008 to VS2010) and cannot get the error using VS2010 SP1.
The reason for this is simple : the Page Init is *not* now being raised at design time.

Are you using the SP1 version?
There's a lot of chaff in the link you provided. AFAICS the basic problem was that VS2008 did not fire the Init() event at design time but VS2010 did (but I can't check that) ; VS2010 SP1 doesn't.

>The reference to a session variable is in a method called from the OnInit method. I finally figured out how to use the workaround in the post, but I access a session variable in this way in many forms. I was hoping maybe just a change in the web.config would handle this.
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>Anyway, thanks for the feedback.
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>>Is your reference to session in the OnInit method? Did you try the work around suggested here? I have not encountered it but most likely I do not have any cases of session in the OnInit. Can you put it somewhere else?
>>Timothy