Hi
I bought a 42" GT30 about 5-6 weeks ago, have always owned Samsung LCDs upto then and like you, was put off by image retention.
I haven't got Sky so don't watch a lot of sports but I do have a couple of games consoles which I play quite a lot for a few hours at a time and haven't had a single problem. Some games have static images as well so if anything bad was going to happen I'm sure it would have by now. I think as long as you are reasonable about it and don't leave static images on the screen for hours and hours you will be fine, I've gamed for 5-6 hours solid with no probs.
Regarding image retention, as far as I can gather this is not the same as screen burn which used to plague plasma TVs. I believe that even if you do see image retention (which as I say, I haven't yet), it will only linger for a matter of seconds.
I trialed a number of TVs (LED / LCDs included) before settling on the Panasonic and I am really glad I went for it, it's easily the best TV I've owned and image quality is very important to me. I am still blown away by the clarity of image and the colours are really vibrant without being too glaring and over the top.
As the other chap has stated there are maybe problems with GT30s but to be honest if you look hard enough (and trawl these forums enough!!) you will probably find a fault with any currently available TV, I don't believe any set is 100% perfect. However, as I say I am totally happy and have yet to find a single problem with mine.