When Apple announced that they were going to kill off the Xserve, many enterprises clamored against the decision and wanted some sort of solution for running servers. Apple did offer the Mac Pro server. While this is an option, it is not truly mountable within a standard 19-inch rack, which is the standard within enterprise data centers.
The newest rumor is that Apple may be preparing the next refresh of Mac Pros. One of the models in this refresh, Mac Pro Server, may be coming a step closer to being the true replacement of now defunct Xserve by offering rack-mounting. M.I.C. Gadget is the one reporting this rumor. The addition of rack-mounting may finally satisfy the enterprise market . This update would only apply to the aforementioned Server version of the Mac Pro and not to the other Mac Pros in the line.
In addition to being rack-mountable, there is also a rumor that the new Mac Pros will contain a custom processor that has been developed specifically for the Mac that has not been seen in any PC. However, it does not rule out that it may potentially be produced for the PC at some point in the future.
The third tidbit regarding the Mac Pros is a bit more troublesome for enterprises. The rumor states that the ability to hot-swap hard drives may be disappearing from the Mac Pros. Hot-swapping, for those who may not know, is the ability to replace an entire hard drive without shutting down the computer to perform the replacement. This may not be a problem for most Mac Pro owners, but this can be a deal-breaker for those contemplating purchasing a Mac Pro Server. The Mac Pro Server market requires the ability to keep the server running with as little downtime as possible and eliminating the ability to hot-swap hard drives would significantly reduce the uptime.
The last potentially irksome rumor is that the power supplies are not going to be interchangeable. This means that you will have to buy an Apple power supply if yours decides to take a nose dive. This is the most problematic for users since Apple tends to add a bit to the price of their hardware to maintain their margins and buying only a replacement power supply from Apple is not going to be cheap.
Of course, we have no idea if any of these rumors will come to fruition. I would like to see a rack-mountable Mac Pro Server option. Many end-users may not be affected by the Mac Pro refresh, but many enterprises will be eagerly waiting to see what new features are offered, besides the requisite speed-bump, with the new Mac Pro line.