I don't expect the bottom to fall out of them, but don't expect them to appreciate either. I think the folks that have 5,1 models with dual processors will probably hold the best value, but they were the most expensive when bought new.
Assuming the third-party products like GPUs, etc., don't dry up, they should stay relevant for some time.
I am certainly glad that I purchased mine when I did as the new model lacks several features that are important to me. The internal expansion is important as well as the PCIe slots and the optical bays. It has been my favorite computer that I have owned, and I have owned many.
One concern is the support for Apple displays - I feel the LED Cinema Display is only being manufactured because of the Mac Pro, and that market is probably so small right now they don't move many of the displays. I expect Apple to cut them as soon as the new model is released. I hope the next displays (I expect an iMac-type thin model) will be backward compatible with mini DisplayPort as well as Thunderbolt to support the newer screens.
I look forward to using my Mac Pro every workday and it is a pleasure to use.