@TRich mentioned: the official Microsoft pronunciation used by most people in the field is "sequel."
Ah, that probably explains it then! Users of Microsoft's product probably call it Sequel, while others call it S-Q-L. Microsoft didn't invent the standard so they can't set the standard pronunciation for all vendors.
@rstone: Yes, and while we're at it, don't forget to pronounce the name of the USA PATRIOT Act as you-ess-ay-pee-ay-tee-are-i-oh-tee, because that is also an acronym (it stands for Uniting and Strengthening America by Providing Appropriate Tools Required to Intercept and Obstruct Terrorism -- look it up)
For the rest of you: if you want to look like a clueless n00b, then by all means, pronounce it ess-cue-ell, but the official Microsoft pronunciation used by most people in the field is "sequel."
Actually, people like you bother me. SQL, as jrotenstein pointed out, is an abbreviation for Structured Query Language (S.Q.L.) and pronouncing it as sequel is a bad habit that should not be adopted. So, please refrain from talking about topics that you know nothing about, and if you are a D.B.A. then God help the company that you work for.