Any iPhone app on the iPad should play fine with iPhones. But that may not be what you are asking - as for iPad native apps playing with iPhones, the coolest one I've seen so far is IsoCards... iPhone comes in about 1 minute into the video... Beyond that I know of Cliffed HD off the top of my head. I'll come back with other good ones if I think of them. Carcassonne will be iPad native soon!
I know about the "tabletop" games like scrabble or poker where the iPhone functions as a secret holder while the iPad is the board. I just want to know if when I sit in my house and play a multiplayer game like Asphalt, Uno or Pocket Legends, do I only connect with iPhone/ iPod Touch players or can I have a multiplayer match with someone using an iPad>
Dominion and Dominion HD are fully compatible for online game play. All device types are treated the same in the online matches.