Over here in the UK many dealers already have their demo units received and set up. Delivery seems to be imminent (yes - it's 2 months after you asked the question). As usual the first wave might already have orders just waiting for them. Also the major RIP vendors appear to have their release versions pretty much ready. I'd expect mid December for customer shipments for most existing orders and the ability to keep up with market demand sometime early 2009.
The devices work great though.
Keep in mind that the spectro is not "on-board". It is an add-on which gets mounted to the printer. You can order this add-on either as a "normal" or "UVcut" model ("UVcut" EXCLUDES UV-light from affecting the measurement, while "normal" INCLUDES it - I use CAPS, because X-Rite typically uses the include/exclude terminology in their documentation, which can cause tremendous irritation as actually a DTP41UV means that the built-in UVcut filter EXCLUDES UV wavelengths). All international print standard datasets are using the "normal" filter setting, which you should consider when using the device mainly for proofing to standards.
Either way - you get with both devices the option to measure on black backing or white backing.
Media change - especially of large and heavy rolls - is much easier with the spindle-less design and speed is very high. Also with the added orange and green the achievable gamut covers more Pantone colors than any other Epson before. Matte and Photo Black are in the printer at the same time, which helps if you need to print on newsprint or matte coated proofing stocks or onto materials such as watercolor paper. As the x880 series already the printer has a built-in Ethernet interface for high speed connection. I am not fully sure, but it might well be Gigabit.
They work well for proofing also and several vendors of proof software have their solutions with the x900 series already certified by the respective standards bodies (like FOGRA)