I experienced something odd yesterday.  I rode my bike to work as usual and after I got here it started raining.  Rained pretty solid all day, but not a hard downpour. My bike has sat out in the rain at work before.  When I left in the afternoon, it was still lightly raining.  I started the bike up and it would not stay running.  Had to really stay on the throttle to keep it running and then even after 5 minutes of riding it still wanted to idle down and almost die at traffic stops.  by the time I got home (12 miles) it was running pretty much ok and would idle with no trouble.  It almost felt to me like rain water (or moisture of some type) had gotten into the fuel system somehow.  I wonder if it is possible for rain to have gone in through the gas cap somehow?    I did not ride or even start the bike today yet so I am not sure how it is doing now but my instinct tells me it is fine since it was running fine when I shut it off yesterday afternoon.

I swore I had a rain/humidty/heat related condition for weeks until I determined the vacuum petcock wasn't acting right until the bike was completely warmed up (and sometimes not even then).  This may not be your issue, but try putting it on PRI if that ever happens again.

UPDATE:  This morning was the first chance I had to ride again and when I fired it up it was back to normal.  started on first crank and idled just like it always has.  It had to have been the moisture from the rain.  Thanks everybody.