No, this post will make no mention of Paris Hilton. Promise. (Fully realizing that if I had mentioned her, it would attract an undesirable crowd via Google).
I don't personally have a T-Mobile, but a former coworker of mine recently had a game he developed published on the T-Mobile stack. The royalties he receives from just one game is an eye opener. He can't retire (nor quit his day job) just yet, but it sure is a nice source of extra income. His entire lineup so far can be seen at his site.
It's almost enough to get me more involved in programming for mobile devices. It's like the early days of programming on my Commodore. On many phones, you have roughly the same constraints. I wouldn't be surprised to see a Commodor 64 emulator on a cell phone.
Apart from my limited contribution to RSS Bandit, I tend to be a server-side developer. My last project was to expose my company's platform to mobile devices over HTTP. I didn't spend much time in the client end of it at all.