Despite Venus being very similar to the earth in size and general composition its crust appears to be very different. Earth has a crust made up of moving plates, which collide to" alt="global view of the surface of Venus" /> form mountains, and have volcanoes around their edges. Venus has definitely had some form of vulcanism in the past as there are far too few impact craters on the surface so something must have been destroying them, but until now there has been no evidence for active volcanism happening now, not least because seeing the surface through the thick sulphuric acid filled atmosphere is very difficult.
Suzanne Smrekar and colleagues have been studying Venus using an instrument on the ESA Venus Express mission which can look through narrow infra red transmission windows in Venus' atmosphere. They have particularly been studying structures which look like hot spots on earth – these are places like hawaii which are associated with upwellings in the mantle and lots of vulcanism.
They haven't seen flowing lava, but they have seen rocks which are very good at emmitting Infra red, possibly rocks like basalt on earth. On many planets this wouldn't help very much, but on Venus the atmosphere is so reactive that the high emmisivity minerals such as ferrous silicates will react with carbon-dioxide and sulphur dioxide to form a thin crust of minerals like calcite, or quartz dolomite on the surface, which are much worse at emmitting infra red.
They don't know how exactly long this would take, but experiments indicate that it could be in the region of a few years. So it looks like Venus might be the first rocky planet other than earth found with active vulcanism. It is also important because understanding how venus works helps us understand the earth.