0. Make a compressed air rocket launcher!! See description for a link.
1. Improve the design! Native Solidworks 2012 files attached, along with a PDF drawing.
2. Slice and print. I used Cura 12.07 with default settings. 0.2mm slice height. 190 degrees. The base uses the full height of the Ultimaker, so it might have to be adjusted for a different printer. Printed in a vertical orientation, though the thumbnails here show it horizontal.
3. Clean up print if necessary. These were pretty clean for me, but I used a dremel to clean up any little overhanging materials at the top and bottom that would create interference on assembly or on the launcher.
4. Assemble the top and bottom. I heated up the bottom of the top piece to soften it a bit, and applied some cyanoacrylate glue to the top of the bottom piece, then slid them together.
5. Prepare for launch! I pumped up the chamber to 80 psi.
6. Stand back!
7. Fire!
8. Admire flight, and/or collect shattered pieces for forensic study!

Possible improvements:
a. I would be interested to know if ABS is any more durable than PLA. Calling all Makerbotters and Reprappers!
b. Solidify the nosecone a bit. On my one successful rocket, the tip busted open to a ~4mm hole after the first flight.
c. Open up the inner diameter a bit? Its a bit tight on the 1/2" Schedule 40 launch pipe. Maybe if it was opened up a bit, more energy would go in the vertical direction rather than toward explosive destruction.

Photo & Videos: flickr.com/photos/cbonsig/sets/72157630700622384/