True, except in that Pentium is an English neologism, and thus is pluralized like an English noun. Don't get me started on the plurals of octopus or virus, please.
Well, if my Latin knowledge hasn't abandoned me -um is the ending for masculine words whereas -a is used for feminine words. I suppose the proper plural of Pentium would be Pentii :p
Yeah, you're right now that I think about it (the example that reminded me is pluralizing walrus to walri). Personally I do this sort of thing just for personal amusement (as I imagine this persons boss did) rather then actual correctness.