On my phone as I'm typing out a word in T9 it shows a pulldown menu with possible choices and at the bottom it has 'Add Word?' where you can add a word to the T9 dictionary. It also will predict common phrases so if you type in like "I'm heading out" it might fill in the rest of the way with "to the bar."

But this phone runs Windows Mobile Smartphone Edition so I think its T9 is smarter than the average T9. Text entry on previous phones I've had was rather more cumbersome.