Not yet - I think that is the future MS is heading towards, but we're not there. What I do like about having both is the consistency of the interface - live tiles, horizontal scrolling apps, People Hub, etc. Its a very clean and seamless transition from phone to tablet to desktop when its all MS. The other advantage is that both the phone and Tablet/Laptop/Desktop are built around MS services, Skydrive, Office, etc.
Android and iOS have gone the route of unifying tablet and phone with a separate desktop/laptop GUI. Microsoft, so far, has gone the route of unifying the desktop and tablet with the phone having a separate GUI - although I think the goal is to unify all three.
One Note app surprisingly works/syncs beautifully between Windows Phone 8 and Windows 8 on Surface Pro. Used to dread server based... i mean, "The Cloud" (especially after "The Cloud" put a closing/freezing bug in Portal 2), but am warming up to "The Cloud" after using One Note.
With Windows Phone 7.5, accidently discovered it could be used as a wi-fi router between Win7 desktop and Win8 on Surface Pro. Thought it was the ethernet cable, but turned out the ethernet cable wasn't working and it was the phone doing the routing (so comparable speed, without the numbers).
Windows Phone 7.8 with Win7 or Win8 is kinda a pain because you have to use a program called Zune to transfer files back and forth, but with Windows Phone 8, you just plug in the USB and you drag files between folders.
Xbox accounts are synced in someway between Xbox, WinPhone and WinDesktop, but to what extent, i don't know.
Calendar sync wickedly between WinPhone7&8 and Windows 8 desktop too.
The Surface Pro AC adaptor has 1 plug to the socket, but an extra female hole too so you can plug both the Surface and WinPhone into it at the same time...also, you can plug a WinPhone into a Surface Pro to charge the winPhone on the go (although draining the Surface Pro battery to do so).
Well Windows 8 like to 'see' Windows 8 phones a little more than Windows 7 does. With 7 ( at least a few months back), you needed to preload the Nokia Software to get the phone ID'd and connected to the PC. With my Pro and a Lumia 820, I simply plugged the phone into it and it went and installed everything and even synced the phone to the PC for me, well it asked me to sync it, but it did everything but actually Sync. I dont think its so much the Surface Pro as it is that Windows 8 is a little better at the newer stuff.