Phones with slide-out keyboards are definitely available on AT&T! We have about 4 right now.
Samsung Propel
Samsung Impression *Brand New this week*
Pantech Matrix
Pantech Matrix Pro
Also be on the lookout for the LG Zenon (Xenon? I have no idea how they're gonna spell it) - it should be out in the next week or 2.
*edit* The LG Xenon officially went on sale on our website today for $99. Touchscreen and a full keyboard! Very nice!
Is it true that the Impression is the new Eternity? How are the reviews? Is it better than the Eternity? How is the Xenon shaping up?
The Impression is like a sibling to the Eternity only with a slideout keypad. If you have not seen the screen, then you need to! It is huge and gorgeous. It's the first phone with an OLED screen (geek tern, I know) it basically means it is nice and bright and can be seen very well in the sun.
Phone Scoop is my favorite review site. They had this phone over the weekend and posted their very in depth review today. Since it is so new, we haven't had much personal time to spend with it, although I will get 2 days to carry one for myself next week.
The Xenon is a smaller version without the really nice screen. The bright blue color is a turn off for most adults, but it functions about the same as the LG Vu if you have seen that phone before. The touch screen is not nearly as nice as the Eternity or Impression.
The $10 data plan others have mentioned is if you already have a family plan with unlimited texting. They give you a $5 discount on data. Otherwise it's $15 a month.
Wow, I typed a lot, but there's a lot you need to know to make a decision!
Oh, and BTW, I'm not gonna be pushy like everyone else, but I really love my iPhone

Just so ya know my stance on that one too
Happy hunting!