Had my first round of extrusion problems on my PrintrBot Simple Metal, and now I’m older and wiser (well, older anyway).  Turns out the problem ultimately was as simple as adjusting the filament feeder gear tension, but I didn’t know that at the time.  So, I carefully cleaned out the extruder and hot-end tip, using some tips I picked up on the net.  A couple of things that weren’t on other posts, but worked well for me:

1. After removing the extruder tip from the extruder assembly, I temporarily remounted the extruder on the Y-axis arm and reconnected the heater and thermocouple plugs, and then ran the temp up to 200 or so. With the extruder tip missing, any extra filament material came out the bottom in a fairly large drop.
2. With the particular extruder assembly I had with my recent version (July 2014) PrintrBot Simple Metal, the top (filament feed) end of the assembly is cool enough to blow into, pressurizing the filament channel.  This allowed me to simply blow extra molten filament material out and confirm that there was a complete, open passage through the extruder assembly (minus the 0.4mm tip at this point).
3. After blowing out the extruder tube, I screwed the extruder tip back on the (still hot) assembly, and let it heat up.  Then I stripped  a length of 30 gauge wirewrap wire I had laying around from a previous lifetime (30 gauge is about 0.3mm) and used it to clean out the now-hot tip.  This take a bit of fiddling, as solid wirewrap wire isn’t very stiff, but it did the job nicely.  In fact, I was able to strip a 4″ length and push it through the assembly from the hot-tip end  all the way up through the top, verifying that the passage was completely clear.

After doing all this, I made some test prints, only to discover that the filament was still not feeding properly.  Eventually I tracked this back to an improperly tensioned feed roller bearing assembly.  When I added some more tension, the filament started feeding perfectly.