3937 edwards,
At the 250 hours mark common opinion says that the tv is pretty much run in by now, 200-300 hours is the norm for a run in period although some sources recommend 500 hours. At 250 hrs you are slap bang in the middle of the recommended run in but if you feel as though you want to continue with your run in settings until you reach 500 hrs then do it but i don't think it's of any value tbh.
As for the image retention this will happen from time to time but will ease within the life of the tv, you have to remember that 250 hrs usage is very little when compared with the life of the tv and you should expect the issue to settle further with more use. It's easy to get rid of anyway so i wouldn't worry just yet, i experienced the same as you with my 42GT30 and i was of the opinion that upping the contrast and brightness from the run in settings was to blame.
I calibrated the screen accurately but i still got some image retention, however, this did start to settle down after a week or two and now (after over 1500hrs) i don't get any image retention issues atall so i would say the key to this is just time and do bear in mind the tools at your disposal on the tv, pixel orbiter and scrolling bar. I found them very useful for a few weeks after the run in but i don't need to use them anymore.
Hope that helps.
audiowizard,
You are taking the right precautions with pixel orbiter and scolling bar but i would recommend some calibration of the screen because black, contrast and colour levels do need some tweaking in my experience although THX is the most accurate but even that required some tweaking after my run in period.
To check the hours on your tv you need to turn to google as i can't provide a link here as it's against house rules i'm afraid, just search with the following 'check the hours used on your plasma' and you will find a thread on another forum about it but be careful, the volume buttons can change values whilst in this mode and read the instructions carefully before you start. You should also be aware that by going into this menu the settings can be wiped to the factory default although this didn't happen to me.
Best of luck and hope that helps.