LCD = Higher Resolution and more Light Output and are not affected by high altitude.
Plasma = Currently, overall better black levels with over 10:000:1 contrast ratio common. However, there are ery few options in true HD (High Definition) 1920x1080. (Pioneer and Panasonic being the only two brands to date selling true resolution. Most Plasma TV's scale 1080i and 720p resolutions using a 1366x768 panel or a 1280x1024...etc. You get the idea.
LCD - Most look terrible at night in a theater environment. They have a symptom called torch mode. If you've ever sat in front of your computer in a dark room using an LCD panel to watch a movie, you'll understand what I mean. If you decide to go LCD, Look for a brand like SONY Bravia that uses an adjustable backlight (a MUST have, in my opinion). If you are viewing at night, you will want the ability to turn the backlight down. The fatigue coming from an LCD panel in torch mode in a home theater with controlled lighting is the biggest reason for people not liking LCD.
Plasma and Burn in. A mis-understood issue for the most part. If you walk into Best Buy, some sales dweeb will tell you that Plasmas have burn-in. It is caused by low persistance in the panels ability to respond to light and dark levels. Watching 4:3 images on a widescreen set were the most common reason for burn-in. However, this issue has been solved by the leading companies and Burn In no longer exists on 2007 leading models (like Panasonic and Pioneer) for the most part.
Recommendation:
Money no object / Controlled lighting : Pioneer PROHD (wait till April for the new 1080p 1920x1080 set). The PRO FHD1 is by far the best looking flat panel on the market and can be had over the internet (with careful shopping) for less than $5K.
Money no object / difficult lighting: Sony Bravia LCD or SONY SXRD Rear Projection. The issue with rear projection is bulb cost and you WILL have to replace the bulb in a DLP or SXRD set. Enter this figure as part of your purchase budget. Bulb cost can be higher than $700 installed...and bulb life can be less than 3 years.
On a Budget / Controlled Lighting: Panasonic Plasma. I read the comments above and for the most part agree with the assessment made by other members. However, Plasma is not dead. Unless you can purchase an LCD at a reasonable cost with adjustable backlight, I would pass on LCD. Sony is signficantly higher than other brands. I would expect new models of Toshiba and Mits. to have adjustable backlights by summer.
On a Budget / difficult Lighting: Toshiba 1080p LCD.
Hope this helps.