I personally saw Best Buy run an ad for a 50 inch plasma for $3999.00 several months ago which is a phenomenal price on a 1366x768 display. Bottom line is they were discontinuing that product in their stores and wanted them OUT. When I worked for a local A/V company, they would regularly lower prices to below cost just to get the last 5 or 10 out the door. Yes, they take a loss, but it gets money in and old stock out the door that may otherwise just sit there forever.
I actually ended up getting that 50" plasma on eBay... FROM BEST BUY (yes, same one) - for $3200.00 and I picked it up myself at the warehouse. They rolled it out, brand new in the box to me.
Absolutely - when Best Buy gives you a good deal, go for it!