Sounds like it's your signal / cable provider, especially if you don't see it on DVD or Blu-ray. Compression is used in the transmission of video, and it often causes poor image quality in dark scenes.
Compression wasn't really used for analog TV, so you didn't see those sorts of artifacts in the past, plus old TVs have a much lower resolution than HDTVs today... the consequence is that a good source looks great, but any flaws in the source are much more visible.
That being said, some sets do a better job of handling poor quality sources than others, but I think you'll see the problem you're describing with pretty much any HD set.
I have a new Samsung LN40A650. Whenever I am watching television (not Blue Ray or DVD) any night scenes are terrible. They break up into pixels or color blotches. Is this an inherent defect of LCD technology? Of the Samsung? Any analog television I have ever had greatly outperforms my Samsung.