Well, what an interesting turn of events.  Skype has announced Skype for SIP (click here to join the beta)... What can it do for you?  Ok, I 'lifted' this from the Skype site, but it lays it out neatly...

With Skype For SIP, your business can:

• Receive and manage inbound calls from the 405 million Skype users worldwide on SIP-enabled PBX systems, connecting the company website to the PBX system using Skype click-to-call buttons
  
  
• Place calls via Skype to landlines and mobile phones worldwide from any connected SIP-enabled PBX, saving your business money with Skype's low rates
  
  
• Purchase Skype online numbers to receive calls to the corporate PBX from landlines or mobile phones
  
  
• Manage Skype calls using your existing hardware and system applications such as call routing, conferencing, phone menus, voicemail and call recording and logging - no additional downloads or training are required
  

Tom Keating wrote about a solution that allows outbound calls to the Skype community here, and this can be built on to make outgoing calls if PBX / Phone manufacturers can build the logic into their own products (more on this later as I'm going to try something out in the lab 1st - maybe with a Mitel PBX and the 3CX software based PBX)..

Now, a couple of things...

Firstly, this service will only support G.729 to start with?  Why not G.711 at least? and what about HD Audio? - please let this be the SIP Trunking service that 'kick starts' HD across the corporate trunking world.

Secondly, as I haven't tested it yet it will be interesting to look at how the SIP trunking connection from Skype works, what features it support and I'm hopeful it will be good as they have 'good intentions' re: working towards the SIP Connect recommendation from the SIP forum.

And... if you want to read other peopls comments on this, check out this great thread at Gigaom

So, it's very early days here but all of a sudden the SIP trunking market has just gained 'one heck' of a player.