On February 10, 2009 at 16:56 UT, Iridium 33 (24946, 1997-051C) was in collision with Cosmos 2251 (22675, 1993-036A). A large number of debris objects has resulted from the collision.
Iridium 33 was an operational satellite in Orbital Plane 3 of the Iridium constellation, while Cosmos 2251 is believed to have been out of of use for some years.
Iridium 33 is no longer functional. The long-term effect of the debris on the rest of the Iridium constellation is as yet unknown.
While there were three on-orbit spare Iridium satellites available in Orbital Plane 3, the replacement of the destroyed satellite by one of the spares was delayed for some time, presumably until the likely effect of the debris cloud had been assessed. However, in early March 2009, Iridium 91 (27372, 2002-005A) [note that some sources still label this satellite as Iridium 90] was raised to the operational orbit to fill the gap.
satellites collide in orbit and the discussion
threads on SeeSat-L:
Space collision starting at http://www.satobs.org/seesat/Feb-2009/0213.html
Two satellites collide in orbit starting at http://www.satobs.org/seesat/Feb-2009/0214.html
Iridium, Cosmos collide starting at http://www.satobs.org/seesat/Feb-2009/0215.html
See also the official Iridium statement regarding the collision and loss of Iridium 33: Update on Iridium Satellite Constellation
Dr TS Kelso has analysed the resultant debris cloud - Iridium 33/Cosmos 2251 Collision