For the past year, yahoo hasnt changed the GUI of its IM software that much . but now they have released a major BETA release couple of days ago.
The download is approx. 11 Mb (as compared to MSN's6 MB). As soon as you install and use it , you will notice the interface has dramatically changed. Download it from here :-http://messenger.yahoo.com/beta Click read more for screenshots.

Chat window:-
Logon Logoff alert :-