Skype for Windows upgrades a bunch of back-end and minor features aimed at improving user experience, most notably tweaks that should improve call quality.
For users who regularly suffer from excessive background noise or dropped calls, the changes to the audio engine should cut down on those problems. Vista users who suffered from their screensavers turning on during video calls should suffer no more.
Also, incoming requests will no longer display profile images, a boon to those who mix their Skyping with their office hours and don't want that NSFW image of their hottie's avatar popping up. Other notable improvements include updates for the Firefox and Internet Explorer plug-ins, better native configuration for firewall and router settings, and a fix for the bug that was causing Skype to crash during file transfers.
An upgrade for the Mac version of Skype is due soon.