On my two year old iPad 2 I ran the image retention test and it failed miserably (no joke). I ran the test for 5 minutes and I left the brightness at the default settings (~40%). I've had this unit for over 2 years and it's been used a ton. I've never noticed any image retention issues or was negatively affected by it. My guess is most IPS screens suffer this issue to some degree and it's not noticed in real world usage scenarios. You can probably chalk this 'issue' up to blogger and media hysteria.
http://www.marco.org/rmbp-irtest.html
http://www.marco.org/rmbp-irtest.html