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Glitch causes virtual keyboard to disappear during texting and other applications, users say.

From InformationWeek:
According to posts on a popular tech forum, the update, officially known as Windows Phone 7.5, causes the Windows Phone virtual keyboard to disappear or otherwise act up. "Am I the sole one having this bug after the Mango update?" a user posting under the name Arturobandi asked on a forum maintained by XDA Developers.

Apparently he's not. Numerous other posters on the forum said that they too experienced the problem, and other keyboard-related issues, after upgrading their phones to Mango, which Microsoft released to the general public last month.
  Windows Phone Mango Breaks Keyboard, Users Say