A priceless little quote from an anonymous post from another message board quote in TNH's Making Light:

Don't say "sci-fi". It's "SF". The community does say "sci-fi" sometimes, but pronounces it "skiffy". Explaining the social nuances involved is beyond the scope of the present work.

This is from a set of tips on attending SF conventions, but it's good to remember during any interaction with skiffy fans and/or professionals.

(For a bit more on the nuances, see Paul Brians's "Common Errors in English.")