Terrain engine details and some nice references here:
(you may need to register to read it, but I can highly recommend it and it's free). If you want to program games, there's an awful lot of useful information at GamaSutra.
I'm doing my games as a side-line hobby, 'cos I'm pretty sure I'll always earn more doing what I do than being a games programmer, *especially* on a platform that's not really designed for gamers.
If you want to make money then maybe it's worth considering targeting the ARM based Gameboy Advance... but the problem there is getting a publisher.
I'm messing about with mine simply 'cos I like (where's the 'fawning' smiley when you need one?) ARM processors. iPAQs, etc, make better toys than development boards with no screen and limited upgrade potential.
I would *guess* that the games that have sold the most are aimed at middle-aged businessmen with no lives (thus, Pocket Strip Poker, SimCity2000, boring damn Golf games).
Looking at Handango, it seems that SimCity2000 has been downloaded 29511 times ($30 shareware - no idea how many people actually registered, unfortunately)... which surprised me (very much!).
... and that's just from Handango.
Hrrrrm... maybe I should try and do more to my games after all!