Thanks for the input David.
I think you may be on to something but let me add some detail if you don't mind to get your opinion if there's a work around.
We have a join page and on the page there is a checkbox a user can select to subscribe to a mailchimp list. if the box is checked and the user clicks the join button the code behind does a bunch of other stuff and then runs listSubscribe() and then response.redirects
to the user's personalized home page.
If we comment out the response.redirect the listSubscribe() works fine and if we look at the mailchimp list we can see the user has been added. If we keep the response.redirect the user is not added to the mailchimp list.
The biggest noticable difference is that without the response.redirect the "join" button click causes the join page to postback and of course with the redirect it doesn't as it leaves the page.
The only mailchimp related code on the page is the listSubscribe() and the assemblies of course.
thanks in advance for any thoughts you may have.