The way Apple thinks, an Xmac will be a top of the line Imac with 4 cores or mini or if they really had there way an ipad pro. Seriously, they don't seem at all like they want to open up a new line of desktops for any reason whatsover, quite the opposite.
Why so much gloom?
Dell has announced, but supplies no shipping date.
HP has announced, but ships only in the US.
Until March there was nothing to update, the AMD 67xx / 68xx GPUs offered no discernible performance gain, http://www.anandtech.com/show/3987/amds-radeon-6870-6850-renewing-competition-in-the-midrange-market same with Intel's offerings.
What do you think Apple developers use? Apple will keep updating the Mac Pro because it has a high margin, costs relatively little to update particularly if they keep the existing case, and because they need them internally and have a good market externally. Apple developers to use Hackintoshes! I don't think so.
Also the last thing Apple needs right now is less products. 60% of their revenue comes from a single product (the iPhone), they need more products like the much rumoured TV. Look what happened to Nokia (debt downgraded to junk status this week, once the world's largest mobile phone maker).
The Mac lineup just needs the updated components and it will keep growing due to more people adopting the Apple ecosystem. Adobe are making it easier for Mac users to switch to Windows with CS6, as for the first time you can install on either or both. There is just no incentive for them to upset a lot of already edgy professional users.