As far as i'm aware Kenny's thread relates to tvs with power problems that are 2 generations older than the tv you are considering, for example my G20 has had no such issues reported that i can find. I wouldn't let it put you off but that's a personal decision and you must consider that the internet is awash with negative comments on products and very rarely is there much praise to be found, it's just the nature of the internet and forum sites i'm afraid as people are more motivated to complain than praise.
If you are still happy with the performance of your Z80 then don't change it but if you think the new model offers enough in terms of performance and features when compared to your Z80 then go ahead and get one. You are much better off trying to audition one before you buy if you can so that at least then you can judge for yourself if it's worth upgrading.
I am changing my G20 to a GT30 in the next few weeks after seeing one in action although i had to travel to a Panasonic dealer to get a proper auditon as the likes of Currys, Comet etc had an awful feed on their tvs making it impossible to judge the picture quality. For me the jump in quality from a G20 to a GT30 was great enough for me to upgrade but the VT30 was a lovely tv and more stylish than the GT30 (wish i could afford one myself but sadly it's out of my price range
) but picture wise i couldn't notice any difference so get out there and judge for yourself, it's the only way