In that case, and I have just tested it on my iPad, the message should have been,
"The username you entered is not available. Please pick another username".
If you didn't see that message I need to know the exact steps you followed. Please can you send that to me via the Contact webmaster page:
www.steveharley.com/contact.html
Update: I have done as you asked and looked at the registration page and I cannot reproduce the problem that you experienced. Unless I can see what you saw, I cannot fix it.
So unless you can tell me exactly what you did to get this problem, there's nothing more I can do.