This is a tough one. First, there's some considerable differences between Libya and both Iraq and Afghanistan. Concerning a legitimate authority, Libya is the only one of the three to show that there is a resistance force capable of self-governing at least parts of the country. If it weren't for this, the resistance itself would not be legitimate. The second side of the legitimacy issue is who authorized the "no fly zone." In this case, it went through the UN with a vote of 10-0 with China and Russia abstaining. That is pretty substantial. This is the sort of multilateral effort that was lacking certainly in Iraq and to a degree in Afghanistan.
Considering just cause, the waters get a little more murky. Sure, defending unarmed civilians is a great cause, I'm just not sure if it merits millions of dollars worth of aircraft, logistics, and cruise missiles. The word "genocide" has been tossed around as well. I admit not knowing a lot about the population of Libya, but can it really be considered a genocide? Are the resistance fighters of a specific race or ethnicity? Not sure. "Massacre" or "bloodbath" would seem more accurate. Do we have a responsibility to prevent massacres and bloodbaths? I suppose so. But this no-fly zone seems to have been pushed more by European nations than by the US. France especially appeared the most hawkish. Which begs the question. While a blog isn't an academic publication, I do like to cite when I'm able. This next tidbit I can't though. Probably some NPR show -- Europe is more dependent on Libyan oil than the US because Libyan oil is chemically better for diesel (Europe uses a much higher % of diesel than the US). This might be a small cause and would definitely be unjust, but I think there's a bigger issue at hand -- refugees and immigrants. Continental Europe already has huge problems with Muslim immigrants. France and Italy especially. Given these two countries' geographic location compared to Libya, I could see this being a greater cause than any aversion to massacres. Just cause, in my opinion, is not met.
Last resort is a toughy. In a situation that is so rapidly developing and amorphous, it's very hard to tell what other options might have been pursued. It definitely felt like Gadhafi was on his last leg about a week ago. Could the world's leaders have either persuaded or coerced him to step down when his position seemed bleakest? Possibly. I suppose we'll know in 15 years when "True Hollywood Stories" takes up the issue. Last resort is going to have to be a push for me. Not enough information on hand right now (unfortunately I don't have to time to research it any more).
So, is the action over the weekend in Libya justified under the "rules" of Just War? I'm not really sure. I'd lean towards "no." But I am heartened at the multilateral cooperation and specifically the reliance on the UN. In this respect, I can state confidently that whether or not this action is justified, (and this next part admittedly is irrational -- in a "yes" or "no" question you can't have "less of a no") it is more justifiable than Iraq or Afghanistan.