Although it's not a built-in application, it looks like I can remove one of my "Final Four" iPhone applications from my list. Scenario Poker (US$10) has announced that they're developing an iPhone-capable version of the Texas Hold'Em game for the iPhone. An online preview is available.
Scenario currently makes a Texas Hold'Em widget for OS X that's been in Apple's top 50 downloads since it's introduction in April 2005. While I'm not a big fan of Dashboard and Widgets (until they're standalone and multi-threaded, that is) Scenario's Hold'Em Web 2.0 application has a lot of potential on the iPhone
The game is Texas Hold ’em limit poker. It takes ten minutes to learn and a lifetime to master. You’ll be playing in a three-round, shootout tournament against up to 27 simulated opponents. When you run out of chips, you’re out. But if you win everyone’s chips, you keep advancing to the next table until you’re the champion!