There is not a definite answer to this as it depends on how one defines a revolution. However, one distinction I keep seeing being made is between a civil war and a revolution, with numerous commentators discussing whether the situation in Libya is more a revolution or a civil war. Hard fact is that most serious revolutions involved periods that were also civil wars: France, Russia, and China for starters.
I think a useful place to start, if not necessarily to end up, is to consider the view of that old revolutionary, Karl Marx. For him it involved the uprising of an oppressed class against a ruling class, these defined in terms of control of the means of production. A full-scale revolution involved a change of the mode of production in his terminology, along with the replacement of one ruling class with another. The archetypal model for Marx was the French Revolution, in which the bourgeoisie replaced the landed aristocracy, and capitalism replaced feudalism, although there was Thermidor and reaction, and Napoleon declaring himself Emperor and naming new aristocrats, with the Bourbon Restoration following. So, maybe it was not so successful after all, even if in fact Napoleon went around Europe smashing feudal institutions all over the place and did create the modern nation state of France. As it is, we may be stuck with Chou En-Lai's reply to Henry Kissinger regarding the outcome of the French Revolution, "Too soon to tell."
What is going on in the Arab world looks to me most like what came in Europe in 1848, an international uprising with some similarities across nations as well as differences, although in the short run a failure, if not in the longer run. In some countries the ruler is a monarch, but so far none of those have been overthrown. Tunisia and Egypt were essentially military dictatorships, overthwrown by would-be democrats, although we need to wait and see what will happen, with some ugly anti-women and anti-Christian demonstrations in Egypt. As for Libya, recent developments suggest that Qaddafi may not fall after all. Too soon to tell.