Well, not EXACTLY...
I happened upon this blog at MSDN this weekend that was linked from an article that claimed Server 2008 completely killed Vista SP1 when configured as a workstation.
http://blogs.msdn.com/vijaysk/archive/2008/02/11/using-windows-server-2008-as-a-super-desktop-os.aspx
It's supposed to be the exact same kernel. How is this possible? Being ever the willing experimenter, I had a fresh ISO of Server 2008 Standard so I figured I'd give it a whirl. I had installed Vista SP1 a couple of weeks ago on my Dell Precision M65 (Core2 Duo 2.33, 2GB RAM)and, while much improved over Vista RTM, it has been sluggish when pushed to do simultaneous tasks that XP could handle easily.
So I figured what the hell. I installed 2008 last nght. Holy crap, this thing screams. It's as fast as my XP was before. Apparently the default server install doesn't install any of the crap you don't absolutely need. I had to install the Vista version of my video drivers, start a couple of services to enable audio and desktop themes, and enable support for "desktop experience". Now I'm even running aero with no noticible reduction in performance. All apps that installed on Vista SP1 install on 2008 too.
Also, the annoyance of not having server admin tools for Vista is no longer an issue, you can use the "remote server administation tools" to administer 2003 AD, DNS, WINS, DHCP, etc.
I think I'm going to keep running this for a while.