I forgot to post this, but I had the issue again today so I was reminded!
I posted on this thread back in January about how the Broadcom issue on SBS with ISA was driving me crazy. Well, since then I simply avoid Broadcom cards and still will.
However, one of the comments on that thread from Victor in Dell (thanks!) mentioned that disabling the Receive Side Scaling would fix the issue. I posted back that this did not work, but that was prior to installation of SP2 for Windows.
I can confirm that doing this on a post SP2 machine does indeed fix the issue on the Dell servers I have tested today.
More on the same problem on the SBS Blog