I face this problem all the time because my old Google Talk ID, that has most friends, is associated with a Gmail account while the newer one is on Google Apps.
With the new Google Talk Chat Badge, you can interact with people who do not have a Google account. You need to be a Google Talk user to create a badge, and you need to be online in Google Talk to enable people to chat with you.
Imagine a scenario where you are logged into Google Talk but don’t want friends to know about your online status.
When you mouse over any name in the Gmail standard message view, it pops a small window that will lets your write an email to that person, start a new chat session or even find “recent conversations” with that person.
Call your gtalk or Google Talk friends from Yahoo! Messenger or the new MSN / Windows Live Messenger. You can also make phone calls to traditional landline numbers just like Skype.