I recently visited a speciality Idli joint. Its called Udipi Idli House and is located at King's circle (near the flyover) and you can reach them on 3246 0111 for more details. This is the only place in Bombay that serves authentic Idlis - the way they are served in Mangalore(its a rice pancake & is a steamed dish) as it serves idli's with the right mix of rice and urad dal, most place in Bombay serve plain rice idli's. Idli is a very popular south Indian dish of Bombay.
Its a fast food joint which serves a variety of idlis. This includes regular idli, khotto, moudo, in addition to other exotic varieties. The price of all varieties is very reasonable, but the place is usually quite crowded and you might have to stand & eat your dish (plus its a very small joint). They have two other branches which are - Udipi stores, near Matunga, Central railway station and Udipi Shri Krishna Boarding, LBS market Bldg, Matunga station. There is no branch outside Matunga.