I was worrying about the same thing before I went from a CRT to the 50PZ80 in October.
Looking at the energy tracker chart within my Eon account, it shows that I used on average approx 10kWh per day last month - compared to approx 7.5kWh in Nov 2007.
Obviously it's not exactly the most accurate way to measure the power consumption of just the TV - plus I also recently purchased a new Sony surround system. And the wife is always at home now as she's currently on mat leave.
HDTVtest shows a calibrated 50PZ81 uses 300W.
My PS3 uses about 200W.
My Sony AV amp + active sub uses ~ 280W.
Hence the big jump in my usage.
Read the comment from DunklerStreiter re power consumption of his 42PZ80:
Panasonic TH42PZ80 / TH42PZ80B Review: 42" Viera Plasma HDTV
Yes I am paying more electricity now, but I think it's well worth it.