An excellent crowd-sourced map on Google on the uprising in Libya has been created by one Arasmus, here. It's useful in trying to sort out all the various reports, to get a sense of the ebb and flow of control. Here's what seems to be happening: the eastern cities are protester-controlled, but Tripoli has at least temporarily been bludgeoned into submission and is saturated with pro-regime forces (update, the NYT reports barricades still up in some neighborhoods), other western and central towns are reportedly under attack by military units, and now Qaddafi is contemplating how to regain control of the east before his authority completely unravels. The regime seems to have a shortage of reliable forces, as the army is reportedly divided along tribal lines. (My very uneducated reading of a list of Qaddafa and allied officers in Mansour O. El-Kikhia's Libya's Qaddafi, pub 1997, suggests that they were then concentrated in about six or seven of the army's 45 armor and infantry battalions, although it might not be a comprehensive list).
A few units (maybe Khamis al-Qaddafi's 32nd brigade?) appear to be loyal, a few units (in the east) have mutinied, but the rest are presumably in limbo -- they have not actually mutinied but the regime does not want to commit them, as they may well mutiny as soon as they are ordered to fire on civilians. Libyan opposition websites are confidently predicting the defection of entire tribes, which may be an exaggeration, but the diplomatic defections do suggest that there is a major breakdown of the regime's legitimacy. Hence, Qaddafi needs to supplement his loyal units with mercenaries recruited from sub-Saharan Africa, although probably they would need to be backed by some Libyan armor. The mercenaries are reportedly being flown into airports outside major cities like Benghazi, with the intention of marching on the center. There are unconfirmed reports of mercenary attacks on the smaller eastern cities of Darna and al-Bayda (Ben Wedeman from CNN, coming from Egypt, seems to have made it as far as Tobruk as of 12 GMT without encountering any pro-regime forces - Darna and al-Bayda lie between Tobruk and Benghazi). This suggests that the regime might be trying to subdue the rest of the country before tackling Benghazi. If the protesters in Benghazi have obtained enough heavy weapons and organized a serious enough defense, they may be able to hold out for some time. Syria in 1982, with a much larger and more ideologically and ethnically cohesive regime defense reserve, took weeks to subdue the similarly-sized city of Hama.
The question now is whether or not an international body (NATO, the UN) can declare a no-fly zone. Given the size of Libya, the fragility of the regime, and the apparent dependence of the government forces on air supply, this may not be as toothless as it first sounds. A no fly ban (if it is enforced) could complicate the assemblage and the supply of mercenary forces, and avert an offensive against Benghazi that might lead to tens of thousands of civilian casualties. Even a few more days of respite for Benghazi might see more tribes (and more military units) drift into the anti-Qaddafi camp -- one presumes that if the repression is anywhere as near as brutal as it sounds, many officers will soon be hearing of the deaths of close relatives. On the other hand, any sort of foreign intervention would reinforce a regime narrative that Libya is under attack by outsiders -- the Egyptian experience suggests that that xenophobia is the embattled despot's best friend -- and could lead to many unforeseen complications, particularly if Libya slips into a prolonged civil war.