its shows me that domain is not avaible, but it's. Where can i write

Register the domain somewhere else (GoDaddy/eNom) and point it to Google.

This query led me to do some experiments on Google Apps

I tried choosing a domain name from common names, here are the names & the results

wsj.com Allowed to book
gtalk.com Allowed to book
ebay.com "Already registered"
rediff.com "Already registered"
yahoo.com "Google Apps does not currently support this domain name."
microsoft.com "Google Apps does not currently support this domain name."
msn.com "Google Apps does not currently support this domain name."

:)