real-time form validation checking username availability

You can also use the same functionality through cf7 + jquery, jquery have really good example on their site.

However, cf8 would also do, like Azadi mentioned.

pure cf8-based solution will be using CFCs and cfajaxproxy tag.
you can do it in pure js, too. it all boils down to sending an
asynchronous request to a page/cfc which looks up the data in your db
and returns something to the calling page which tells you if te name is
taken or not.

if you have never done any ajax stuff, you may want to learn the basics
and then how cf8 does it.

google is your friend in all the above.

Azadi Saryev
Sabai-dee.com
http://www.sabai-dee.com/