It all depends. If you want to get your programs on carrier decks you are either going to have to learn J2ME (Java) or Brew. Java powered phones are by far the most popular, including all Nokias, S-E, and RIM Blackberry models.
However, I think the iPhone platform is the future. You have a distribution model that bypasses the carrier decks with iTunes. Which is huge because it's a pain in the ass to get on carrier decks, seeing how most NA phones are locked from installing software unless you download it directly from the carrier's portal. Then you have the fact that Apple users tend to open their wallets more for software.