The Mac Pro workstation could be seeing a long-awaited refresh that could be announced at next week’s World Wide Developers’ Conference (WWDC), according to sources.

Based on an inventory check of the Apple online store, many Apple Stores throughout the United States no longer have any stocks of either the entry-level quad-core Mac Pro or their significantly more expensive 12-core counterparts.

This could indicate one of two things: the Mac Pro has met a resurgent wave of popularity that have strained the distribution channel, or Apple is deliberately clearing up stockrooms for a long-awaited update of the workstation computer.

Major outlets like the Grand Central Station Apple Store have quoted an availability date of June 9 for the Mac Pro server model, while the other versions of the computer will reportedly be ready for pick-up on June 12. Other retail locations quote a June 12 pick-up date for fresh stocks of Mac Pros.

This, significantly, is just a day after Apple CEO Tim Cook is expected to kick off WWDC with a keynote that is certain to cover updates to the company’s two operating systems: iOS 6 and Mac OS X Mountain Lion.

The Cupertino-based computing giant is rumored to be gearing up for a massive refresh of its Mac lines, and the Mac Pro could be the final piece of the puzzle.

Considering that the last update of the Mac Pro was in 2010, and that these machines – definitively at the top-end in Apple’s desktop range – don’t even have next-generation Thunderbolt connectivity, Apple could figure that these computers need an update.