Liam Clearly recently used VSTS to test SharePoint and ran into a bug in the web test recorder. See his post here.
The bug is that the recorder drops requests if the content-type is not set in the response. In this case, the page redirects to another page, but the initial response (the redirect) does not have a content-type header set.
In the resultant web test, it looks like the recorder simply dropped the form post parameters, when in fact the entire request was missed.
We have built a QFE to fix this problem, which is scheduled to be available later this week. We've seen the bug crop up on other sites as well.
Interestingly, we had tested SharePoint before we shipped, but this regression was introduced pretty late in the cycle and we did not have this test case covered in our automated suites that we run in the end game.