Strange dotted screen at start-up

After getting a dead video card replaced at the local AppleStore my iMac now exhibits a strange black dot pattern on white screen just before the log-in window at start up.
The mac was returned to the store, video card replaced again, no change, they decided to replace the
logic board. The dot pattern during start-up now appears intermittently, so I photographed it and found the pattern is the same every time.
• See the pattern here: http://dl.dropbox.com/u/14311256/iMac%20dotted%20screen%20%3F.JPG
The iMac seems to work OK so I am reluctant to return it to the repair shop and be landed with yet another bill. I paid £318.00 for the logic board job but feel this may have been an unnecessary expense.
Q1. what can this dot pattern mean?
Q2. Does it indicate a serious problem?
Q3. Should I just live with it?

I've got the same sort of issue and may have a solution. I noticed it would do it more regularly on startup if my Time Machine external HDD was connected and switched on before powering on the computer. By chance, I forgot to do this a few times and in those circumstances, I switched the external on after the login screen...no dotted white screen before login!
As a result I've done this ever since and no screen issue whatsoever.
It may be that the drive is confusing the iMac during it's startup process, although why that would affect the graphics, I don't know. It works for me, though.
So if you have an external connected all the time, try it without it for a few weeks, only connecting it after you log in.
If you don't have any external peripherals switched on and connected while you get this odd screen, then I'm sorry, I don't know what it could be.
Let us know how you get on, as I'm sure there are many of us out there who would like to know what gets rid of it and what doesn't.
If your Mac works fine other than that, I'd say it's just a glitch in the startup process that doesn't affect the computer. When I was getting the screen you describe, the iMac would operate normally, so I don't think it's anything to worry about.

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