Skype is opening up its interconnect options for PBX's:  

Skype for SIP Open Beta

Skype for SIP is now in open beta allowing businesses to complement their existing SIP-enabled PBX with Skype functionality.

Overview Benefits Requirements Support PBX Vendors Pricing Get it now
Enhance your PBX
Skype is taking business internet communications to a whole new level with Skype for SIP Open Beta. Businesses can complement their existing SIP-enabled PBX by integrating Skype for SIP without the need for new hardware or software. Any business can now configure their SIP-enable PBX to enable Skype calls to be made directly from their desk phones at Skype's great low rates.

Also, businesses may choose to purchase separately, online numbers wherever they are located to let customers in that country call them at local rates. By adding click to call buttons on web pages and emails all registered Skype users worldwide can reach your business, reception or call center for free using Skype.

The free web-based Skype Manager™ centrally manages all aspects of Skype for SIP Open Beta set up - from creating a SIP Profile to allocating Skype Credit.

[From Skype for SIP Open Beta - Product Overview]