[Galaxy S I9000 General] xpcomputers:
Burn-in would be permanent damage to the screen. It would be caused by those pixels being used more and "wearing out" quicker, and would be visible as those pixels being degraded by always displaying less intensely than those around them. Hence being visible as a ghost image of the signal strength, battery meter, time, AM / PM etc.

Memory effect (assuming it really does exist - as I suspect) would appear to be the same thing, but only on a temporary basis. i.e when the time and signal strength meter etc. has been displayed for a while at the top of the screen, and you shift to full page of a suitable colour, you would still faintly see the time still in that corner. However how visible it was would depend on how long you'd recently had the screen on for, how brightly, and how recently ago those screen pixels had been used.

The main difference between this and burn-in would be that leaving the device/screen off for while would reduce how visible the ghosting was.

So:

If what we were seeing was only memory effect and no burn-in, then leaving the device off for a while would cure it completely.

If what we were seeing was only burn-in, then there would be no difference based on recent usage... the ghosting would remain at least the same, and in fact slowly get worse each week or month.

If, as I suspect, both have an effect, then there is slowly increasing permanent damage occurring, but also, it will look even worse when the screen has just been used brightly. Leaving the screen off, would reduce a proportion of the damage.

Since I personally mostly use the screen at minimum brightness (and even have it extra dim at night using "screen filter" app set to 10-15%!), mine does definitely have some permanent damage, but might have a lower proportion compared to someone who uses the screen on full brightness all the time. So using the time as an example, I can see the current hour clearest, and if the screen has been off a while, my ghosting is quite dim and blurred (but still visible).

Someone who has more major damage, might not be able to see the difference between memory effect, and burn-in, as their burn-in would be too prominent.

First time a new phone is used, it shouldn't have ANY burn-in (unless factory testing left some), but would still display some memory effect.

Over time, the permanent burn-in would increase to the point where it was ultimately FAR stronger than any memory effect. In between these two extremes, is where I suspect most of our devices sit, except for the heaviest users!

This is my experience and hunch, but I haven't been able to definitively prove it.

Oh, one final difference between memory and burn-in. Memory effect will be overall crisper and influenced by the most recent usage ("A" or "P" will show more strongly based on which was used most recently.. and hour might be crisper than minutes), whereas burn-in will always be more blurred in areas that change (i.e. all time will be very blurred) and only the most permanent features will be crisp (i.e "M" in AM/PM will be crisp).

Be good to hear from others if they can prove that they get a mix of both types of ghosting, and if we can get an idea for how long the memory effect lasts.

Ironically, one good place I can see the ghosting is on the light brown splash screen for the XDA app!

Ideas for testing:

Leave screen off for a while (longer is better - maybe overnight?). Turn on, and immediately open the XDA app, and see the level of ghosting in the top corner. Note how blurred the hours are, the minutes are, the A / P is, and the M is. etc

Then use the phone for an hour solid, and test again, and check for the same blurriness.

After that second test, if the ghosting is overall more prominent, and you see the current hour and the A or P particularly more clearly than before, then you have just proved you have a mix of memory effect and burn-in.

I expect there to be a range of answers depending on how old the unit is, and how well used it has been (most used screen brightness level etc), so when reporting back, please include than info.