It turns out a new driver for the Broadcom NICs dated 9/29/2009 fixed the problem.  I now have a functional 2008 R2 Hyper-V cluster.  And as a bonus, on 9/28/2009, they released a driver that actually makes the Intel Quad VT card I started with work in R2.  Yay, 14 ports.
I have two R710 servers with Windows Server 2008 R2 Enterprise.  Each has 4 Broadcom LOM ports and 1 dual port Broadcom NIC for a total of 6 network ports.  (They also have two dual-port Intel NICs but those are dedicated to Hyper-V vswitches.) The Broadcom NIC ports are all connected to separate networks.   At any given time, five of those six connections can successfully work.  By work, I mean can ping the switch and other devices on the same network.  The other connection times out.  If I disable/re-enable the NIC with the failed connection, it begins working and one of the others takes its place failing. Any ideas?
R710 - only (n-1) of n NICs can maintain active connections.