Supplies of Apple's Mac Pro and Mac mini server lines are reportedly constrained, indicating a refresh is likely in the near future. According to 9to5 Mac, a source indicates these two product lines should see updates within the next month or so. 9to5 Mac doesn't give any info on the current supply status of the non-server versions of these two Mac models, but it's a safe bet that the standard and server lines will see updates at around the same time.
We already expect to see a refresh of the MacBook Air line relatively soon, and the Mac Pro and Mac mini are due as well, having last been refreshed nearly a year ago. It's a given that all of the forthcoming Mac model refreshes will include Apple's new Thunderbolt I/O port, and the Mac Pro and Mac mini are also likely to have their CPUs updated to Intel's Sandy Bridge architecture.
9to5 Mac also notes that Apple's AirPort equipment supplies are still constrained, which may indicate that those products are still due for the refresh that many (including us at TUAW) expected to see at WWDC. With Mac OS X Lion due for release next month and a bevy of product refreshes expected soon, it's shaping up to be a busy couple of months for the Mac.