I'm proposing a retag as: [queuing-theory] --> [queueing-theory]. You can vote for it here:
http://cstheory.stackexchange.com/tags/queueing-theory/synonyms
Quoting from Wikipedia:
The spelling "queueing" over "queuing" is typically encountered in the academic research field. In fact, one of the flagship journals of the profession is named Queueing Systems.
Please comment and vote as you see apt.
queuingis the correct form, since the finaleis dropped when the word takes-ing(except in special cases). That's why there are more results forqueuingthan there are forqueueing. However,queueing theoryis a stylized term for this field in academia, and so I think we should respect it. Anyway, your last sentence tells everything: "[we] should make one of the spellings a synonym of the other." – Sadeq Dousti Jan 10 '11 at 16:21