From Ars Technica:

The other new feature from Skype is currently being tested by a handful of Skype partners. Skypecasts are massively multicaller conference calls supporting up to 100 people from anywhere in the world. Each call is moderated by a host who can control who gets to speak via “mute” and “eject” buttons. Skype is positioning Skypecasts as an online community-building tool and a way for people to “discuss shared interests.” Plans are for the current testing to be followed by a wider public beta.

A conference call with 100's of people involved is an interesting “large group” problem. With the ability to have an integrated visual interface (AKA Skype's interface), it seems to me that Skype has a big opportunity to create cool visualizations of the group interactions. You don't want to hear the fidgeting and mumbling of 100's of people, but what if you could SEE it in the interface?