Dimly lit room: Plasma, LCD, or DLP

I would go Plasma, LCD doesnt do blacks as good as Plasma and that contrast is where I feel the rubber meets the road, but lamps and such may reflect on a glossy Plasma screen so thats something to consider if you use lights in room at night.

If you dont have issues with lights (windows and such) and want to use for movies and non casual 12 hour per day use with many on and off cycles get a Projector, huge screen and surprisingly cheap cost makes the Projector a winner in the right circumstance. I set my 90in HD Projector system up for less then $1000.....thats way cheaper then any LCD or Plasma or even DLP rear unit and gets out of the way for music use.
If you do think a LCD monitor is best way to go pay attention to contast and hz rate, as Theo noted get 120Hz unit and the larger contrast the better as that is the difference between the blackest blacks and brightest whites.

As stated above all 3 work great in a dim environment. 120hz is only a factor if you watch a lot of live sports..other than that the difference will not be noticeable if you buy a Tier 1 LCD (Sony,Sharp,Mits,Samsung). The video processors are good enough in these 60Hz models..I doubt you'd see a difference for the extra cost of 120Hz.

Blacks will be better on a Plasma as a whole, but there are some trade offs that plasma lovers do not mention.

Though the blacks are a little deeper on my plasma, my LCD has better shadow detail. In other words, on dark scenes I see more detail on objects like buildings and the side of a person's face that is away from direct light.

This is also important and easily visible when you do a side by side comparison on the same material. As time goes on a plasma will grow dimmer(it will start to lose its brilliance).. an LCD doesn't.

A plasma is more susceptible to burn in... LCDs are not.

So you really have to look and decide what is most important to you and where to make a compromise.

Neither is perfect..both plasma and LCD have wider viewing angles than DLP. LCDs look best when viewed directly or from a tighter viewing angle than a plasma, similar to a DLP. Though not quite as critical as a DLP.

Pick your poison.

Plasma TVs are very energy inefficient. LCD TV are much greener.

A 42" class plasma will consume about the same amount of electricity for 3 to 4 hours of daily use as a modern full size refrigerator consumes in 24 hours. The WSJ says to add $200/year to your electric bill for the purchase of each large plasma (this obviously varies by location and usage). LCDs are more efficient, but still relatively high energy users. Roughly a third less than a plasma. Old-school CRTs are by far the more energy efficient.

LCD/Plasma power usage:
http://reviews.cnet.com/4520-6475_7-6400401-3.html