I'm currently running WordPress 3 beta 1 in network mode.
Have W3 Total Cache installed for a few of the sites which works like a charm. But there seems to be a problem with the CDN connection to S3.
My S3 account works and has been tested. If I manually create a bucket it 'will' upload the files there. But it's failing when requesting the data.
On this line when I use test upload (error for actual upload is similar but different line):
354 status.addClass(data.result ? 'w3tc-success' : 'w3tc-error');
Firebug says: Data is Null
Now as I said, it does actually upload, but because it's not getting feedback it's hard to keep track of what works and what doesn't.