I used Domain Mapping plugin on this one to allow the client's to configure themselves, everything was working okay, minus cross domain cookies not working, until about 2 weeks ago.
One of the sites that was running Adwords to one of its pages, got flagged for spam. I immediately scanned the network, check wedbmasters, etc... and nothing. Everything is clean.
I raised the issue with Google and they complained about a script that is part of WPMU's domain mapping plugin, specifically this: