It disables the entire transparency within the vista styling engine and changes it to the Vista basic theme (the equivalent of those who just meet the minimum requirements for Vista and those on Home Basic OS flavour) until you close iTunes and/or Quicktime (depending on whether either or both are open).
According to so many forums I have looked into, it's common as hell. There is a really small proportion of people who claim not to be affected by this bug which Apple do know about but are still yet to fix it despite versions 7.2 and 7.3 being released being classed as Vista compatible.
EDIT: amazingly I have found a link that apparently gets around it! for anyone interested
this link explains how to get past the bug and gives an explaination too.
This is gay though and Apple should have addressed this is as a fix as standard. I don't know if it works though I shall report back in a bit.
I've also found Limewire does the same Aero disabling thingy but haven't found a workaround based on the above linkage.
EDIT #2: the fix seems to work so far! just a shame limewire is still being gay though