Good Idea, shouldn't be too hard to code provided Bondings has made a list of the subdomains somewhere in mySQL
It's checked after submitting, you'll get an error when it's taken or not allowed. In that case, just hit the back button and try a new one. But if you pick a normal subdomain that is longer than 3 characters, then the chance is very low that it is not allowed or taken.
Bondings always has a solution either in place or almost ready.
Just to be obtuse, any change of some AJAX magic so that it tells you if it's available as soon as you've finished typing?
I might be adding some ajax features, but definately not in the soon future, as tidruG mentioned. I would rather like to have a lot of functioning features instead of only a few fancy ajax ones. Besides, the subdomain changes aren't often used anyway, about 1 a day