Doing adult mobile development for the past year, I can say from a development perspective, it sucks ass. Check out the WURFL and the varying number of handsets and caveats for each, for example.
In regard to WAP, having checked out a couple of competitors, I thought very little of their mobile products (maybe it's changed since I last looked, though): two examples - the payment gateways are cumbersome and their download pages are a bunch of links expecting the user to know which screen size and media type their phone supports (Always remember users are dipshits and lazy, keep it simple). Also, it could just be me, but I think buying a photo or video is more of a novelty than anything else, especially on such a tiny screen.
In regard to SMS, yea, adult SMS is still screwed in the US. If anyone knows how to get a US carrier to approve a shortcode catered to porno, let me know.
Furthermore, after the cost to partner with an SMS aggregator for messaging and the chunk the carriers take out of each one, you're left with pennies for premium rate SMS.
Reglardless of my grumpy feelings toward the mobile space, I think the most money will come via interactive services where you can interact back and forth with a customer ( and charge a premium rate for each outbound for an interaction ), combined with engaging them to a WAP portal that offers additional, cheap upsells to content of some sort. I think bikini / NN people in this industry have a huge wealth of content they can use in the US market that will get approved by carriers and be enjoyable to customers... Say a "chat" service that's non sexual where the chat client can send a "current" picture (and more for +$$) of themself to the user participating in it -- all for a minimal fee.
Dunno... Though I'd share to pennies.. *shrug*