Cosmos-2421 naval reconnaissance satellite of the US-PU/Legenda system (29247/2006-026A), launched on 25 June 2006, stopped performing its regular orbit-correcting maneuvers at after February 10, 2008. According to Philip Clark, a long-time observer of the Soviet and Russian space program, the satellite suffered a breakup during February 15-16. As of March 20, 2008 NORAD registered 15 fragments of Cosmos-2421 (32710, 32715-32728).
The satellite was reported to experience problems with one of its solar panels right after the launch. It was, however, able to operate for more than 20 months - not much less than other satellites of the US-PU system, which usually operate in orbit for about two years.
Fragmentation at the end of the satellite service life is also a common occurrence - of the last five US-PU satellites (Cosmos-2347, Cosmos-2367, Cosmos-2383, Cosmos-2405, and Cosmos-2421) only one - Cosmos-2405 - produced no registered debris.
Cosmos-2421 may be the last satellite of the US-PU system. This system and the Tselina-2 electronic intelligence system are expected to be replaced by a new system, known as Liana. The first launch of a Liana satellite from Plesetsk using Soyuz-U launcher is reportedly scheduled for 2008.
UPDATE 03/29/08: 20 more fragments have been registered - 32732-32748, 32752-32754.
UPDATE 04/15/08: Five more - 32758-32762.
UPDATE 05/21/08: Debris are still being registered - the last one as of today is 2006-026HD.
UPDATE 07/27/08: As of today, 426 pieces of debris were registered - the lat one is 2006-026SV. 270 objects (including the original satellite) are still in orbit.