If you’re in Europe or other locale that uses a comma for decimal points, you might be disappointed to find the comma disappeared after upgrading to Mango.
Unfortunately, try as I might, I can’t bring it back (although I have raised the issue with Microsoft). In the meantime, if you need a comma, you can tap-and-hold on the period to bring up the option. Also, I’m adding an option to DeekFit Gym 3.8 to bring up a more full-featured keyboard instead of the digit keyboard.
I apologize for the inconvenience – hopefully this is addressed in a future update of Windows Phone.