Ok, Khalsa's made some mistakes...
1) It's ui_messagemode and ui_messagemode2
2) I'm not sure if it matters, but t and y should be lower case. At least, that's how I've been using it and it makes sense. Tremulous might not care.
So again, all of the input text stuff is fixed by doing:
/bind t ui_messagemode
/bind y ui_messagemode2
It's not that what we have there now is a bug, it's that this is what the client will have as default in the future. As a bonus, you get slightly different prompt text which is more illustrative of the purpose.