Dynamic form field evalutation

Interesting and a valid answer.
However, if someone wants to brute or map the db she can try.
As long the db is set up properly (escaping), the username and password inputs are sanitized and both have appropriate length the attacker can hack himself to death because Apache too does a pretty nice job to prevent things like that... correct?

The only remaining thing to fear here is a quantum computer or worse.... a cluster of it.
Then we are all through and you'll never know when it hits .-\
Sometimes totally paranoid I dream' about it in conjunction with nano-tech...

Nice scenario right?

I'd find it rather annoying to redirect the user every time he has chosen a username just to try again until he finally found some valid one.

Although I appreciate your engagement I don't like to ask for help to find a solution just to be confronted with another problem.
That's not very productive.

Any suggestions?

The only absolute knowledge attainable by man is that life is meaningless. (Tolstoi)