In-store availability for the Mac Pro has long been spotty at Apple's retail locations, making it difficult to read too much into any apparent shortages as hints of future updates. But MacRumors has learned that Apple began recalling Mac Pro stocks from its retail stores several weeks ago, with retail store staff having been instructed to inform customers that they may still order the machines online.

A quick survey of roughly 100 U.S. Apple retail stores for availability of the entry-level stock Mac Pro model shows that nearly all of them are indeed out of stock for in-store pickup, although we did find four California stores and one Texas store with machines available for immediate pickup.

Projected availability dates for orders placed online today vary significantly, with some stores citing availability on June 8 or 9 while others would not be able to offer a Mac Pro for pickup until June 12 or 13. Apple's keynote address at its Worldwide Developers Conference takes place next Monday, June 11.

Apple has been rumored to be updating nearly all of its Mac products next week, although it remains unclear whether the Mac Pro will be included in the mass rollout. The Mac Pro was last updated in July 2010, and new Xeon E5 processor options to provide a substantial upgrade to the line are now available.

Separately, we've heard that Apple is holding a secret "overnight" for staff at its retail stores on June 12, with employees being kept in the dark about the focus of the session. A separate overnight session to be held tonight was highlighted last week, but we've heard that tonight's session is simply a routine rearranging of sales floor content.