Ok, so I took my carb out and cleaned it.  The thing was coated with a nasty sticky brown substance.  So I got it all cleaned out, stuck it back on the bike.  Switched gas tanks, gas, etc.  Worked fine.  Went back out a couple days ago, and it's gummed up again!  

Any ideas as to what it causing it?  I believed it to be the gas, but I think I've done all I can to eliminate that as a possibility?  I admit to not being much of an authority on carbs.  Any thoughts as to what's going on?

Just thinking out loud here because I don't know your whole situation...

Is it possible you've overfilled your oil?  When did you last unplug the drain hose from the bottom of the airbox?  Have you checked your airbox to see if it has gunk in it?  In its stock configuration, the crankcase vents to the airbox and if it's overfilled with oil, can blow oil into the airbox.  There is a drain hose (normally plugged) attached to the airbox.  You may try pulling the plug (over an oil drain pan or coffee can, etc.) to see if anything comes out.

I'm assuming that when you say you changed the tank, you made sure that the replacement was clean, along with the petc0ck/filter assembly, fuel line and vacuum line.

This is another place I think a clear plastic inline fuel filter would have been an assist from the start, as it would help filter any problems coming from the tank and you'd be able to see it pretty easily.  A clean filter would turn you away from the tank/petc0ck/lines.

If your crankcase does turn out to be overfilled, please check to be sure your oil doesn't smell of gas.  I've heard of one or two instances where a failed petc0ck allowed the carb to flood and the excess fuel ran into the crankcase.  While this is hopefully not the case, it's worth being sure.  If it is, repair/replace the petc0ck and drain/replace the oil.  If you know your carb isn't flooding or if you just changed the oil and think you may have overfilled it, cool.  When I first upgraded my shocks to the 12.5" Progressives, I didn't account for the changed angle when changing my oil and overfilled it slightly.  I wound up with a bit in my airbox and had to wipe it out and clean my K&N.  It didn't get to the carb.  I noticed it because the plug was missing from the drain hose and it began to drip a bit after a ride.  It was pointed out to me that the plug is a good idea though because it helps to avoid sucking excess dust/debris into the airbox.  I just put a stainless bolt in the end of the hose with a hose clamp to keep it in place.  Now I make sure to elevate my front tire to level the engine when checking the sightglass.