Mobile phones, accessories, add-on apps, ringtones, wallpapers, games, etc. are a pretty good niche, especially with the younger crowd. You could cash in on any of these niche specificities if marketed correctly.
A simple search engine query for "mobile phone affiliate" will yield you many possibilities. However, I would first decide what it is exactly that you wish to sell. Phones? Ringtones? Games? Wallpapers? Accessories? A mixture of the aforementioned? Feel like marketing something a little bit randier? Check out this:
Code:
https://www.sexgoesmobile.com
The possibilities are pretty limitless in the mobile arena.
After determining your product, you should decide upon how you will go about advertisement/marketing. You could build a website, blog, myspace, squidoo lens, etc. Make one of each, who gives a shit, the more the merrier!
Traffic. If you are going to be successful in any market, you need traffic. Get a little spammy at the social networking sites where all the kiddies hang out, do up some read_me_first.txt and add it to some torrents then upload away, make some eb4y advertisements, etc. I am sure you can think up some other ingenious ways of making this work for you.
As far as SEO is concerned, you keywords will obviously be dependent upon your niche. Find some popular sites selling similar items. What are their keywords? Read their source code to find out and get some ideas for your own site(s).
Sorry about the long rant and insertion of the overly obvious if you are a veteran marketer.
