Mobile phones are definitely worth thinking about. There's quite a web-app skew round here, with associated bias towards the big shiny featureful platforms like Android and the iPhone, but don't discount the other 90% of phone users. There's a lot of life in ordinary J2ME, mobile web, and even SMS applications yet.<p>The one big caveat for mobile development is that it's a pain in the arse to monetise. If you do premium SMS billing (so it shows out of their phone bill), expect the carrier to take at least half of the gross price. If you don't, good luck getting someone to enter a credit card number on a 1-inch screen.<p>In more locked-down environments, there's also the issue of getting "on deck", which is to have your site or application endorsed by the mobile operator. This greatly increases your exposure, but it's a long, fraught, gruelling process of direct negotiations with operators, and I wake up every morning saying a little prayer of thanks that it's not my job. Accomplishing this, so fast and so well, is one of the biggest pieces of evidence that Sam Altman is in fact a genetically enhanced superbeing.<p>Like any sort of development, though, mobile development isn't a <i>what</i>, it's a <i>how</i>. But if you have something you reckon people want, and you can fulfil that desire through their mobile phone, go for it!