Printrbottalk.com user Jerrill created some .20mm hotends for the printrbot. Check em out. How low can you go? He also posted a make your own tutorial for the community: http://www.thingiverse.com/thing:53316
Seriously, only use the highest quality filament with nozzles this small. The cheap chinese filament has more debris in it than you might expect, and you’ll start running into it a lot with these smaller nozzles (just recently happened to me a handful of times on a 0.35mm nozzle at the lab)
Also, 1.75mm filament would probably be best as well. The amount of back pressure this thing will produce is likely to cause problems.
Yep, Chinese or not, any filament issues will be a problem in these smaller nozzles. First thing that happened to my .35mm nozzle was a clog due to filament debris.
@Mike_DiMare PLA from @Vik_Olliver is of the highest quality. You can get it state-side from http://printbl.com and in aus/nz from http://diamondage.co.nz - So if you use a nozzle like this…make sure to buy his filament. He’s been there since the beginning of the reprap project.
blush
@Mike_DiMare Thanks for linking to Thingiverse tutorial and my online shop!
How low can I go is a great question! I’ve got 50 more acorn nuts and new drill bits (0.15 mm, 0.10 mm, and 0.05 mm) arriving tomorrow. I’ll be adding other size to the store as I can prove them out with the 3 mm Ubis. I’ll add 0.30 mm and 0.25 mm too. I’ve just spent a lot of time at 0.20 mm just watching it print in amazement. 
I’ve been lucky to have no clogs so far with my filament. I know it’s coming.
So far the only pressure issues I’ve had is a little leaking of plastic around the nozzle threads, but the teflon tape around the threads was in need of repair. I plan on upgrading to a 1.75 mm hot end soon too, though printing slow seems to work so far. And I’ve observed that there is some variation in the quality of the hobbed bolts from Printrbot. Some have teeth that are sharper/deeper than others. Those teeth have to be sharp sharp sharp to prevent stripping out the filament.
I’m contemplating adding a disclaimer on the site that the nozzles are experimental because I’m not into misleading anyone that printing with them is going to be easy. But it sure is fun!