Here's a neat addition for folks running the original Ultimaker printers -- Joris van Tubergen is making his modification for "unlimited" height printing with a Ultimaker available for purchase via Kickstarter.
It's very clever - you just flip the print head so that it extrudes out the top (which will be the new bottom), mount the Ultimaker printer upside-down to the system, and then connect the Z-axis related cables to the system. A 10 minute process!
Joris lists a few recommendations for people using this system, including the use of a larger nozzle to speed up printing, and the use of the Tunell filament monitor to watch out for filament running out or getting jammed.