"Sherbert" heard and seen written in the U.S. is well, just wrong. Perhaps the second "r" is inserted due to the strong influence of the "r" in the previous syllable. Help Wordies - can't find my linguistic dictionary - what is this phenomenon called? A similar historical linguistic phenomenon has occurred in umlauted vowels in the Finno-Ugric languages I believe.
I think it's called something like "Americans talk how they want and f*** the King's English," or something. :)
Seriously, it does have a name, but it escapes me... Not a non-prevocalic R, is it? No wait, that's the phenomenon of my Bronx-born grad school prof adding an extra R to the end of my name when the next word started with a vowel.
P.S. I say "sherbert" just to tick people off. Fight the power!