According to this Business Week article 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 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 – thanks to Hakan Aydin for this pointer!