it all depends on how many people signed upto the gmail accounts as to weather it will be worth it. at least thats part of it.
If I was working for Gmail thats what I'd be looking at. the popularity for Gmail would somewhat determine weather it would be worth making an IM. and if they do make an IM the would certainly mean that alot of people jumped to Gmail.
People like new things, the amount of people I know that use multiple IM clients, it's ridiculous. Anyway, not that they are but if they do, MSN has been ahead for to long, it's someone elses turn, I hope it works out, I'll certainly have ago.
obviously everyone would have to keep MSMessenger for a while till most people convert.
I just hope it has a nicer interface than there actual Gmail accounts if they do lol. i also reckon its gonna be web based and not an actual install.
Either way It's already BIG news