Thomas Baekdal did a great write-up on usability with XMLHttpRequest-powered forms on his website. Highly recommended if you’re building applications powered by Ajax (on whatever language you develop in), because he covers things the untrained eye may miss sometimes, like the disbelief people show when stuff happens instantaneously (which is the point of Ajax in the first place).

One *small* addition to Thomas’s write-up is the need for a global completion label when we’re doing forms with Ajax. He does write about the need to highlight the active element, and to visually indicate data for that specific element has been saved (as you can clearly see on his example), but with no real feedback when someone’s completed the process - in whatever kind of form, not just his example -, the end user will never be absolutely sure.

You should have a look at his article.