Ecuity, a Washington state based company is rolling out a virtual PBX service based on the Intel backed venture, TelSym. This market sector is heating up very quickly.
Already active or announced in the space is Packet8, Covad's GoBeam and Voice Glo, with more sure to follow. But the Ecuity offering, when live, proposes to be available all the way down to the PocketPC form factor, something that when blended with WiFi connectivity, will provide even more ways to cut costs for the mobile professional and maintain connectivity.
I've used Skype on my Pocket PC and other than the tinny mic quality, not an issue of Skype, but of HP, the quality in public hotspots is fine for short chats. TelSym, which was designed for thin client use is likely to finally be able to gain traction. The weakness is that it is not SIP based, limiting features or meaning somewhere in the chain additional software is needed to drive the service offering.
The whole PBX market will be changing. Other companies have different plays on this, including Popular Telephony who are moving in a non server direction. If you're in the market for a new PBX, it may be best to wait until more information comes out.