an answer to a very pertinent question
I don't know about you guys, but "necropolises" is terribly difficult for me to say. So, since this is a Greek word and having some Greek training myself (although it's been a while), I looked it up. I'm a certified linguist (see the shiny M.A. Degree) -- I like doing this stuff.
The Greek plural of necropolis is, in fact, necropoleis. The last syllable is pronounced like "lace".
That's not to say that you can't continue to use "necropolises" if you want to -- that's allowed by the rules of English loanword borrowing. But if you're like the people I know and you don't like how "necropolises" sounds and/or you want to sound fancy, feel free to borrow the Greek plural form too. This linguist says that you're allowed. ;)
(N.B. Some people that I know really like using "necropoli", which seems Latin. With no Latin training, I'm not sure how they would deal with Greek loan words like this. Perhaps a Latin-fluent wow-lady can help!)