I don't think doing this is necessarily a good idea. It depends on your overall tagging strategy, but some of the more established "standards" for tagging, such as this one:
http://wiki.hydrogenaudio.org/index.php?title=Foobar2000:Encouraged_Tag_Standards
recommend *not* using the Album Artist field at all unless an album is by multiple artists. That makes more sense to me than mirroring the artist fields into the album artist field without any real reason.

To a certain extent, the iTunes "compilation" check box does the same thing as what the "Album Artist" field is meant to do on albums with various artists, but if you ever find a need to share albums with other apps (Squeezebox, WMC, etc.), it makes sense to use the fields in a standard way.

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Also, I forgot to mention this, but iTunes, like Windows Media Center, stores the album artist field in the ID3 tag for Band/Orchestra/Accompaniment. So for classical music, the "standard" is to put the name of the orchestra there.

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