Paulinha - useability testing cannot really be automated - you need to test the look and feel of the system as it will be used by your end user. This needs the human touch and an understanding of how your end users work.
I have not failed. I've just found 10,000 ways that won't work" --Thomas Edison
A simple, relatively inexpensive, and useful tool for usability testing is a video camera and a tripod. (Check out the book Don't Make Me Think: A Common Sense Approach to Web Usability by Steve Krug.)
Some aspects of usability can be addressed by test automation or other means of programming. Usability as previously stated includes look and feel. Look and feel in some organizations is further defined and refined in GUI standards. Example: VB ships with guidelines/GUI standards that include attributes / object properties that can manipulated by the developer. Things such as fonts, colors, size of control and placement can be verified with an automated tool. Companies do adopt these and adapt, or - create their own standards.
Usability also includes performance as part of the user experience. That experience consists of performance as response time to user stimulus. Performance can be measured by all the tools. Code can be (and has been) instrumented for purposes of gathering usability-related metrics. Those metrics typically represent errors induced by the user's inability to navigate properly (usability). Other metrics collected involve using the help facilities.
The video cam is to capture the intangible and is therefore the largest part of usability testing.