You will note that like almost all the VoIP players which have commercial services up and running, that Microsoft has put its weight behind SIP, not gone with anything proprietary. You will also note that so far Microsoft has not said they are going into the business of delivering telephone service against the already established players. My guess is they won't, and instead will chip away at the business previously owned by the Cisco's, Avaya's and Mitel's of the world who had the lead in the space for sometime.
As Paul Kapustka pointed out, nothing from MSFT is new, but the hardware is. This follows the proven Microsoft method of develop the software then the API's for hardware, then have hardware come on stream, then release the whole package. My guess is by next month's NXT Communications conference and trade show that Microsoft will have more examples of end to end voice being "demonstrated" then a few months later at VoiceCon, THE enterprise voice conference start publicly shipping their end to end solutions with partners. It's textbook Microsoft playbook and tend to be slightly after the market is there, established by others, but in piecemeal fashion---you need to buy this from them, that from me, oh and that elsewhere. Microsoft's entry will end most of that, as their Office Communicator will make things easier, the same way that Windows Media has made production of streamed and downloaded content easier.
As for carriers, I see this as a watershed moment. Firms like Covad, Level3, Global Crossing, ibasis, Point One, Sprint, AT&T, Verizon are all selling SIP based voice platforms and have their networks voice ready. This forceful entry by Microsoft means that come 3rd quarter/4th quarter the number of Enterprise companies making inquiries so they can make the switch will start to swell. The sales teams will need to be ready to show what they offer and why they're better.
For developers, this also opens the door, as applications now will have an installed platform to ride on, and of course, it's in the application world that the real money will be made.