A slide in a video from one of the BUILD conferences seems to suggest that users would be able to make phone calls directly in Windows 8 without the need of any third-party software. As spotted by istartedsomething, there was a slide of the Windows 8 Metro UI that showed a 'Missed calls' tile with a telephone icon. Remember that Microsoft is expected to complete its acquisition of Skype by the end of this year for US$8.5 billion, so baking a VOIP-based calling feature into the new OS is certainly not very far-fetched. However, a demo video by Windows Live also clearly shows that in the 'People' app, users can call someone through his mobile number.
Suffice to say, even if this feature doesn't end up in Windows 8, it's a precursor of what to expect in the next version of Windows Phone OS.