Where we stand

Well, nowadays at least switching between Eyefinity and non Eyefinity profiles works quite well.
However whenever I return from Eyefinity I have to exchange both DVI-Cables.

Otherwise I couldn't rotate my monitor. Bliss! Maybe a clean reinstall of the Catalysts might solve that?

I also enjoyed a brief time, after either installing new drivers or after my PC was used for some LAN gaming at an so far unknown screen, when I could actually boot up my PC with all 3 screens turned on.

Then I finally felt the urge to Eyefinity some more, and after that the old issue popped up with a vengeance. Now it's not enough to boot with just the Display Port connected monitor turned off, no I also need to turn off my primary DVI connected Dell monitor.

However that might have changed as of today as I again played a bit of L4D2 in Eyefinity and had to do the dvi-switcharoo at back of the card. So my newly primary DVI connected Samsung monitor might be able to stay turned on, as it had been the case previously during the boot sequence? I shall see tomorrow and report!

That however has lead me to believe that the boot issue might actually be a driver issue, strange as it sounds.
Otherwise I couldn't think of a reason why it started happening exactly after I returned to Eyefinity. And not just this once I believe exactly that was the case before once or twice.