I worked for the Apple Store for nearly 5 years. The only time there would be a vacation blackout would be for the following:
OS release - but for retail box sales, which will not exist for ML (major long lines for Jaguar, Panther, & Tiger)
iPhone Release - long lines and activations, oh my. All hands needed on deck for this, big time. New window designs after short close period, new shirts for staff, re-open, sell & activate
new product line release - such as the first Mac Mini - we had a line going through the mall and sold out - same for the first MBA.
The new iPods, while sexy - never generated lines through the store.
If the "all hands on deck" thing is for retail stores, it does not make sense for ML to be the reason, since this is now downloaded through the app store unless they plan on surprising everyone with retail box versions (which I highly doubt).
If this was a blackout for just AppleCare staff & engineers, that would make sense for ML.
I sense a tremor in the force - such as perhaps a new Retina Display iMac release in tandem w/ the new ML... or a new iPhone.
The "all hands" part of the rumor is that "all hands" need to be available at the call center during that week. It isn't a meeting, or an Apple Store. It is a room with lots and lots of phones with headsets, Macs and people with tattoos.
However it is distributed, people are going to need help. Some are going to run into problems while others are going to buy new Macs and have questions about how to install a printer. The people who answer the phones need to deal with both type of customer. That means time for training.