Found this up on Pocket PC Thoughts: the Dasher project (which already has a Pocket PC version to download) uses an eyetracker to determine where you are looking at the screen, and uses probablity to figure out clusters of words that the user is typing. It can type at rates around 25 words per minute and apparently the system can also use a trackpad or mouse for input.

There is more infomration about Dasher in this BBC News article aswell.