According to this Business Week article [1] Intel and Skype have a deal to only activate 10-way calling on machines with Intel dual-core CPUs, everyone else is limited to 5. This has prompted AMD to subpoena Skype over this as part of their anti-trust action.
There are claims [2] that this has been cracked, but the site that they link to has gone (the nameserver records don’t appear in the DNS for some reason).
Update: Details (including assembler decodes) have appeared here [3] – thanks to Hakan Aydin [4] for this pointer!
