Unfortunately the latest Twitter Bootstrap is broken with Windows Phone: The responsive layout doesn't work.
We noticed this when updating the Wensus website. This is how the site worked with a version 2.2.1 (captured from Windows Phone emulator):
And this is what happened after we upgraded to latest Twitter Bootstrap version (2.3.1):
Rolling back to 2.2.1 fixed the issue.
There's a short discussion about this on GitHub dating back to last December. The issue has been closed as the Bootstrap team doesn't officially support Windows Phone. Which is really unfortunate.