Some users of Windows phone 7 have reported issues with keyboard after upgrading to WP7.5 Mango. Issues are mostly about keyboard disappearing. The onscreen qwerty sometimes disappears suddenly when in use or won't appear at all. Microsoft is already collecting detail from the users and a fix should be out soon.
Customer support is working with the individuals reporting changes to
their experience. We are investigating these reports to determine the
root cause of any issues users are having.
The above is the statement from Microsoft given to ZDNet.