Google is not limiting itself to HTML5 applications. Android developers will be able to target the living room in the future by including a separate layout. Google suggests targeting "large" tablets because the "apparent size of the Google TV screen turns out to be only slightly different from a mobile phone's screen."
There are other opportunities. XBMC is a great, open source distribution meant to turn a PC into a television command center. Its core is written in C++, but many of the add-on scripts are written in Python. Perhaps the easiest way to develop content is to create a website that delivers the content in a format that's easy for XBMC to scrape.
Other TVs offer simpler options. For example, Samsung has an API that accepts HTML5 content. It's like building a Web page, but on a bigger screen for someone who is farther away. It's available on some TVs and Blu-ray players. Yahoo has a similar item, complete with a widget marketplace where people can buy your wares.
Not all platforms are as open. Apple TV, for instance, is willing to accept encrypted content that mirrors your iPad screen via AirPlay. It's not the same as writing your own code, but maybe someday, Apple will open up a TV App Store.
Emerging development platform No. 3: Your clothing
It may be made of cloth today, but there's no reason why your garments can't be one of the next great development platforms. We take our clothes everywhere, and electronics are now small enough to be sewn in without being noticeable.
There are already early experiments in garment hacking. Scott eVests and jackets are prized because they were designed to hide wires. You can put your iPod in a pocket, and the wires carrying your earbuds are threaded through channels so that they pop out of the cloth near your ears. They're not constantly getting tangled or misplaced -- unless you forget where you left your jacket.
One jacket from Hammacher Schlemer has a "five-button control system woven into the outer sleeve" so that you can change tracks without taking off your gloves or removing your iPod from your pocket. The buttons lock up after a few seconds to prevent an errant bump from shifting items.