Apparently you have to do a little dance to get Network Browsing to work and you have to turn off the Sidebar (NO!!!!) to help prevent a rather masive Explorer memory leak. I am more than a little disappointed, to say the least. I expected not a finished product but something a little more stable if we're going to be using this build for the next 3-6 months for development purposes.
I don't have access to the bits (no PDC ticket, no MSDN subscription), but for everyone else's benefit, I found some tips for setting up build 4051.
[Tallent.us]
I guess it is possible that MS expects us to install, look around for 15 minutes and forget about it, but I don't think this was the plan.