This is more than just a Samsung Galaxy tab issue. The same thing happens with my Motorola Xoom tablet currently running MCPE 0.7.1 from the Amazon App Store: Cannot type in any sign, chat window or username. My tablet is rooted and runs CyanogenMod 9 with the default ICS keyboard, Swype and Swiftkey. None will work. I even tried google voice input, but still nothing.
(I had the same problem with signs in MCPE 0.6.1 but could type everywhere else with that version)
I've tried several, including one that is called "Gingerbread Keyboard" (but isn't from Google) and another from Google that is just called "Google Keyboard" but doesn't say which version of Android it is from, but none of them will work for me on MCPE 0.7.1 on my Xoom.