Roundabouts are only a good idea if, along with the well-documented traffic improvements they bring, they improve things for pedestrians and bicyclists.
Walkinginfo.org, a clearinghouse for pedestrian activists, has a good summary of the issues:
Modern roundabouts by their design require motorists to slow down typically to less than 25 mph (40 km/h), and preferably 15 mph (25 km/h) to proceed through the intersection. The literature shows that, given a properly designed single-lane roundabout, motorist and pedestrian safety is almost always improved when compared to conventional intersections. Results regarding cyclist safety are somewhat mixed. Roundabouts have fewer conflict points and lower speeds compared to conventional intersections, resulting in a significant overall reduction in the severity of crashes for all users, although the frequency of some crashes may increase. Multi-lane roundabouts present some challenges to pedestrians, thus reducing the safety effects that roundabouts provide.
It appears that the basic design of a roundabout is pretty straightforward and should nearly always deliver the motor vehicle improvements. On the pedestrian and bicycle fronts, however, as Nathan points out, there can be good and bad implementations. Simply lowering vehicle speeds is good, but not enough. Thoughtful, pedestrian- and bicycle-friendly design is imperative.