Yippee!
My simple mistake: after setting up Ionic ISAPI Rewrite for my main site and testing I proceeded to set up the network, then domain mapping.
When the network install gave me the .htaccess rules for the network I <b>added</b> them to the .ini file that runs the rewrite. Doh!
So basically I had the standalone rules followed by the network rules.
Once I realized they were saying similar things I removed the original rewrite block and everything worked!