Shimano stopped making them a few years ago. Road bike riders wanted a bigger cleat and a lighter pedal, so Shimano came up with some pedals that look more like Look's or Campagnolo's.

Luckily, these old road spd's seem to be indestructable, so they should be around for a while. The cleats come with pontoons for road shoe use and last forever compared to Eggbeater cleats. Track racers and Criterium racers still like them for their incredibly high release tension (good for big sprinters) and good cornering clearance (your shoe will hit before the pedal does). Track sprinters sometimes modify them by bolting toe straps on, too, and you can put a longer bolt in the bottom to completely fix the release (have to loosen the bolt to unclip).

Mountain spd's use the same cleat, except for the M-858 pedal that was made for one year. Shimano now makes the M-959 and I'm pretty sure it's back to the old cleat with those.