I've found the source of that problem, for myself, anyways. When you open the form template from where it resides on the sharepoint server, and publish it, something happens to the published template because it's already opened from the WebDav location. Do this: open the form template from the webdav share, save it to your computer, close infopath, then design the form by opening it from your PC. Publish it from there to its intended location and it should open fine in a web browser.
This works in situations where design checker reports no errors but the d@#$ed form still won't open in a browser
Hope this helps! Thanks!
-j