One day I was in CompUSA looking at (overpriced) USB cables. One from the 'main' area (from Belkin I believe) was $23.99. Another in the Mac area a few yards away, exact same specs and same brand, was $27.99.
I studied the packages for a while to find any other difference -- only a 'made for mac' logo on the packaging. There wasn't even a color difference in the cable (such as Mac/iPod white).
Mac users are freer with their money -- less price-sensitive and probably even richer. Sellers know this, and thus there's a 'Mac tax' in retail prices (and disproportionate effort given to creating high-margin Mac-specific accessories).
In addition to ITunes integration and any arty/cultural factors, this freespending tendency also helps explain the paid music download discrepancy.
That said; I want a MacBook Pro. ;-)