Here are some interesting, DIY plans for a depth of field reducer for video cameras courtesy of the always-great Make Blog.
One claim on the site is that it will help your video "look like film." In a way, yes: Shallow depth of field is more common with film than video, generally speaking. Newbies should keep in mind, however, that film and video work entirely differently: a video CCD is essentially like a scanner, while film is a series of unique frames, each with a different pattern of silver halide crystals (think: snowflakes). It's because of this fundamental difference that video (as long as it continues to work the way it does now) will never completely look like film.
Anyway, I would be interested to see how footage using this thingamajig looks, especially in comparison with something like the P+S Mini35, which is a more professional version of the same thing. (The Mini35 is $7500, the DIY thing would probably run you 1/100 of that.) Some guys tested the Mini35 with a JVC GY-HD100 -- I was impressed with the close-ups, but not the night shots.
UPDATE: It looks like the Mini35 will have some competition soon from an outfit called Cinemek. The demos -- particularly the one that begins with the cat -- look good. And they say they're working on one for the HVX-200. Stay tuned.
UPDATE #2: A reader of this blog alerted me to two other commercial options for shallow DOF. One is the M2 -- AKA the Micro35 -- from RedRock. Another is from Dan Diaconu.
UPDATE #3: Yet another one... this time with instructions in PDF format.