Comparing hi-res images from last October, November and February shows that the volcanos on Io turn from red-hot (so to speak) to cool-blue during the period of a few weeks (sort of like volcanoes on earth, except not as red-hot and a much longer period of time).
The latest images from the flyby of Galileo have also shown fourteen volcanos in an area where it was thought were only 4. The area of Io seems pretty-much the same over it's surface, and this area of fourteen volcanos covers about 5 percent of Io, so it can be estimated that there are about three-hundred (280, if you do the math, but it said 300 in the jpl press release, so that's what I'm saying).
Loki, the most powerful known volcano in our solar system was scanned by Galileo's radiometer/photopolarimeter during one an 'eruption' period and it seems that its caldera (depression in the center of a volcano) was covered with lava for four and a half months. The caldera of Loki is four thousand miles, about the size of half of Massachusetts.
Another observation shows a caldera with 'bright whits deposits' containing purer sulfur dioxide than anywhere else on Io.
The Images mentioned throughout