It may be the 13th or 14th. The thought though is that all the top tier OEMs will be announcing new products that week based on the new Xeon chips, so Apple releases on Tuesday because everyone else probably already announced theirs on Monday.
Hopefully big rumors float around the end of this week. I think it'll happen on Wednesday Nov. 14th due to holiday, that way the main home page gets one more day to advertise Leopard before advertising MacPro with Leopard.
I think this statement has some validity to it. With 10.5 having issues that are always inherent with a new OS, I think Apple would want to stabilize the release before releasing the update to the Mac Pro.