I emailed Lego when we didn't find the plaque that usually comes with UCS models (like the R2D2). Here is what I got back:

...it's great to hear that you are enjoying the LEGO Star Wars™ Death Star™ set! The reason that the LEGO Star Wars™ R2-D2™ and Millennium Falcon™ come with a LEGO plaque is because they are part of the Ultimate Collector Series. The Star Wars™ Death Star™ is not part of this series and is actually meant to be a giant play set where the Ultimate Collector Series sets are designed as large display models.

Very disappointing considering it is of similar piece count, price, marked in "exclusives" on their website along with the other UCS, and along side the "hard to find" UCS items in the lego webstore.

Seems like there are other items listed on this Wiki which are not part of the UCS either...