Google has modified the gmail interface so that GoogleTalk can now be integrated with it. More information available at http://mail.google.com/mail/help/chat.html
Is this a good thing? Yes, certainly. The reasons that Google lists are all fairly accurate. Yet, Google ought to provide the option of disabling this feature. The reasons are the constituents of the remainder of this post.
The biggest advantage of email (and regular snail-mail) over other forms of communication is that the recipient chooses when they should read it. I can see an email header and say, "Oh, it's that bloke again! I'll look at this when I have time on my hands." Now, instant messengers don't give you that kind of freedom. Once you've received a message, it might be considered rude if you don't reply back. Sure, you could set a busy icon. But it does not prevent those pestering windows from popping up when you least want them. At last count, I had a dozen friends on my GoogleTalk list. In the future, I may have loads more. I can't be certain that all of them would respect the meaning of a busy icon.
I do not have any instant messenger program installed on my system at work purely for this reason. I remember hearing a story a few days back of a few office workers who'd gathered around a bloke's desk to discuss work, and POP! comes up a message window with some cupid-inspired emoticons. Pretty embarrassing, huh? Imagine the same thing happening simply because you'd logged into your email.
I, for one, would disable this feature on my gmail interface if I had the chance. Else, I'd just stop accessing gmail at work.
It appears that the chat feature can be turned off. Please see http://mail.google.com/support/bin/answer.py?answer=33807&topic=8405
I guess the Google guys have their heart in the right place :)