Phil touched on something, but I wanted to expand on it a little to more fully answer Jon's question.
The dates that Epson prints on the cartridge box, and their recommendation of 6 months life in the printer is based on the material properties of the ink, and the performance level that Epson guarantees for their "Pro" printers. They do not "expire" / stop working after a certain date (as was alleged that some HP(?) cartridges did).
Many people use inks beyond their expiration dates, and some think that up to 2 years is OK, as long as you're willing to monitor the quality of the output (and re-profile if necessary). Phil's advice to remove and agitate the cartridges every 4-6 weeks of inactivity is sound, and additionally, I'd recommend printing a nozzle test or purge pattern (CMYK color bars to exercise all the nozzles) every week (if you're not printing anything else) to prevent clogging and pigment from settling in the lines
I've only had my 3880 for ~3 months, but I'm really loving it. Hope you enjoy yours.
--Greg