Not directly related but I started looking at Skype phones, and also Blueface with Xmas in mind although like Clubman I can't see a solution that bridges both SIP-based systems and Skype at the moment. Beware the Siemens phones, they seem to use a proprietary version of Skype that you have to download from Siemens.
Blueface are currently not offering ATA only connections (gizmos so that you can use your existing DECT cordless with your broadband router for audio calls) - only a package with a Netgear router for 90 euros. If you just want the ATA (ie if you have the router bit already which we do because we are on wireless broadband) then there are RJ11 PC Mac VOIP adaptors on Ebay for about 25 sterling including delivery to Ireland. This allows you to plug your phone into both your landline and a SIP compatible VOIP service like Blueface if, like me, you're a rural dweller and daren't let go of the landline yet (all those poles, all that cable and infrastructure... if we let it go we will never get it back)
I think that the Skype wireless phones mentioned above could prove brilliant, but do we have enough "free" wireless zones to make it happen yet?
Imogen