This design has been superceded by Ron's Compact Extruder Mark II: http://www.thingiverse.com/thing:24219
This is Ron's Compact Extruder modified to sit on the Ultimaker's XY carriage. I haven't tested the max speed, but I am able to print at 100mm/s without any trouble. The main advantage is that by removing the Bowden tube from the extruder, there are fewer messy strings after each print.
Note: This version of RCX is a mirror image of the original. This allows the lightweight stepper motor to be placed as close to the center of the carriage as possible. Additionally, and as a result, the direction of the extruder stepper does NOT need to be changed in the firmware.
Note 2: I've added two OpenSCAD files. The rcx.scad file replaces Ron's rcx.scad. The ultircx.scad is just a wrapper that calls it. (Put all of ron's files in a subfolder called RCX, and ultircx.scad will then know where to find it).
Cut part of the Bowden tube off so that the part remaining inside the XY carriage protrudes on the top by about 5-10mm (total length of the PTFE Bowden tube that remains in the carriage is about 70mm).
Remove two of the long hand-tightened screws that keeps the carriage together. One of them will be removed permanently (it bumps into the idler otherwise). For the one that you remove permanently, reattach the bottom aluminum to the base wood with a 15mm bolt.
Now, attach the Ultimaker Direct Extruder to the top of the carriage, making sure that the PTFE tube protruding from the carriage goes snugly into the hole on the bottom of the UDE. The other two holes on the base of the UDE should line up with the hand-tightening screw holes. Take one of the hand-tightened screws and feed it through the UDE, through the top wooden plate of the carriage, through the bottom wooden plate of the carriage, and then screw it into the aluminum threading at the base. I used pliers to get it tighter than usual.
Use a 20mm bolt to connect the other side of the UDE through the hole where the second hand-tightening screw used to be.
Once attached, a filament of 3mm PLA plastic should feed easily into the PTFE tube directly from the top of the UDE. Woohoo! Direct mounted goodness to enjoy.