The Talk software requires, and integrates tightly with, a Gmail account... which, as of today, you no longer need an invitation to acquire, as long as you've got a mobile phone that can accept a short text message for authentication purposes.
A sneak peek at the new Google Talk interface.
According to press reports, Google is using the open-source IM protocol Jabber for IM, and using the standard SIP protocol for voice. (See, e.g., NY Times, BusinessWeek, Wall St. Journal) They are also offering to work with all other IM vendors to make their service interoperable.
AOL and Yahoo, over to you.
Google Talk: Go on, go get it. You know you want it.