Swarming motility is a rapid (2–10 μm/s) and coordinated translocation of a bacterial population across solid or semi-solid surfaces, and is an example of bacterial multicellularity and swarm behaviour. Swarming motility was first reported by Jorgen Henrichsen and has been mostly studied in genus Serratia, Salmonella, Aeromonas, Bacillus, Yersinia, Pseudomonas, Proteus, Vibrio and Escherichia. Source: Wikipedia
Swarming bacteria have been observed to move (as a group) at around 1cm per hour. The movement was first observed in the 1970's.
It's been seen mostly in laboratory conditions, but seems to be an innate behaviour that presumably has a role in natural environments as well. Its function, and the reasons for its evolution are unknown.