Absolutely, positively not — no changes whatsoever (and if I had, I would have figured it out in the many *hours* of testing that I had done to try and get to the bottom of this).

I was using the same browser/workstation/configuration as was working find without domain-mapping enabled. Using the same configuration as I have now with v3 beta, that works just fine.

The problem is definitely not on the client side, I can assure you (based on hours of testing parallel WP setups). You may want to read up to my longer post above, for a quick overview, as well.