Steam's gaming platform adopts Skype voice technology 'SILK'
Assuming both parties have a decent headset or phone, Skype is generally very high quality, so this should benefit Steam gamers greatly. Connectivity and reliability are said to be improved here as well.
SILK comes at the cost of bandwidth -- between 8 and 30kbps depending on the circumstances versus the previous steady 15kbps. Of course, if you can't afford an extra 15kbps, you've got bigger problems to worry about.
Boot the Steam client now for the latest update.