They cite that the console design infringes on four of LG's Blu-ray playback patents, although only one model of PS3 is specified. Smells like revenge.
Sony back in December claimed that LG was infringing on seven of their patents with their LG range of phones. Sony also launched a civil lawsuit against LG Blu-ray players. Well LG has no problem engaging in a 'tit for tat' war of lawyers.
Sony and LG hold the role of the world's second and third leading TV manufacturers, with Samsung at the helm of the market. It's very unlikely LG actually expect to 'win' against Sony as they are probably just going to enjoy giving them a headache in US courts.
The complaints were filed with the U.S. International Trade Commission on February 4th. The ITC has the power to bar imports of products violating US patents.
Lawsuits Life's Good, eh?