Starting this week, Yahoo is chasing the coattails of Gmail and its integrated chat by coupling email and IM. Now, just by virtue of logging into a Yahoo mail account, you'll be able to chat with people on the YIM network. This could be a definite plus to people who tend to travel a lot and use public PCs while doing so, or want to avoid installing an IM client.
They had planned this move late last year, and have stated that it surpasses Gmail's built in chat in terms of robustness. If you recall, Yahoo teamed up with Microsoft last year to make each others IM apps interoperable.