The ASUS M4A785-M motherboard has a built in Realtek RTL8111/8168B Gigabit network interface. While it worked out of the box with OpenSolaris 2009.06 (svn-111b) it would drop the connection at random points never to return. The logs were silent about the cause.
Found lots of trouble reports on the net including Bug ID: 6807184 rge driver drops off network. One of the later replies to serialized.net » The littlest Thumper: OpenSolaris NAS on an MSI Wind PC summed it up well: "There’s no know work around for this with the native rge driver. However, there is a third party driver “gani” that seems to have fixed the issue for me: http://homepage2.nifty.com/mrym3/taiyodo/eng/"
One of the many great things about OpenSolaris is that you can set up multiple boot environments, that is snapshot your existing setup, with beadm. See Working With Multiple Boot Environments on the OpenSolaris OS for details.
I pulled down the 2.6.4 "gani" driver and installed the amd64/gcc version into a new boot environment. It solved the network dropping problem but it's painfully slow. Writes over NFS top out at 2.1 MB/sec on a gig link. The old rge driver didn't do any better. I may have to get another network card...