Ok, I'll go from the beginning for anyone else:
1. Create new field 'Song Artist'. Should be standard User Data.
2. Create new field 'Song Artist (Auto)'. Calculated Data: =if(IsEqual([Song Artist],),[Artist],[Song Artist])
This will work in the same way as [Album Artist (Auto)] does: if [Song Artist] is blank it will show [Artist], otherwise it will show [Song Artist].
3. Copy everything in [Artist] that is different to [Album Artist (Auto)] to [Song Artist] (select songs, then Right Click, Move/Copy fields, [Artist] -> [Song Artist]). (You could copy this for all songs regardless, although it’s nicer just to do it for compilations so that you can easily see which ones are different)
4. Copy everything in [Album Artist (Auto)] to [Artist]. Again, you can do this for either all songs, or just compilations, although in this case it's easier to do for all songs, particularly if you've used [Album Artist] for anything else (e.g. Album composers), just make sure that you've also copied these ones in the previous step.
5. Customize all audio view schemes to show [Song Artist (Auto)] rather than [Artist].
Now [Artist] will be used as the album artist, which is good for portables, while in Media Center you can still see individual song artists under [Song Artist (Auto)].
When ripping a new compilation, you'll have to again copy across from [Artist] -> [Song Artist] and set [Artist] to (Multiple Artists).
Hope this helps…
I have a Creative Zen, so I dunno exactly what iTunes/iPods are doing regarding compilations.