To use Google Talk you need a Gmail account and until recently you still needed an invation from a friend to be able to open a Gmail account.
This has changed today as you will now be able to get an invitation code through your cell phone. You can subscribe here at Google's site.
One of the reasons we are offering this new way to sign up for Gmail is to help protect our users and combat abuse. Spam and abuse protection are two things we take very seriously, and our users have been very happy with the small amount of spam they've received in their Gmail accounts. We take many measures to ensure that spammers have a difficult time sending their spam messages, getting these messages delivered, or even obtaining a Gmail account (spammers will often use many different accounts to send spam). Sending invitation codes to mobile phones via SMS is one way to address this, as the number of accounts per phone number can be limited.