Courtesy Doug:

A caulk gun? Hey, if it’s good enough for Taco Bell guacamole and sour cream, it’s good enough for a fine and delicate espresso, right?

[ How-To: Make a cheap portable espresso machine - Engadget ]

However, I'm not too convinced. The parts are not detailed well enough for me. If they had the part numbers so I could ensure that I had exactly what was needed, perhaps. Plus I'm not sure that everything he uses is food-grade or has been tested with the type of pressure that is required for espresso. I think I'll agree with a few of the comments and look into a COTS product that I can just stuff in my pack like:

or:

Update: Some site called Firebox also carries this Presso Coffee Maker, but has sexier images and a little movie showing it in action.

Update: Both of these are so tempting to add to my collection of coffee and tea paraphernalia, except I just dropped a couple bucks on a Philips Senseo. Perhaps I'll have to treat myself for my birthday with yet another coffee making device that I never or rarely use...