It's Fixed!
Well, in Parallels 4.0.3540, anyway...
After a lengthy "update" to my virtual machine (Why does it take so long when I'm using my Boot Camp partition?), I tried it tonight, and both my Icom OPC-478UC (USB-radio) and OPC-1122 (serial-radio) worked perfectly. The VM took control of the 478 and the Keyspan USA-19HS, and I had absolutely no problems.
So, for anyone needing USB-Serial connectivity, Get Parallels 4...
Larry