Bryan McGrath is quite right; current operations in Libya are geared not toward creating a “no fly zone,” but rather towards establishing air supremacy. The difference between the two is that the former would deny Gaddafi the use of the air to support military operations against the rebels, while the latter enables coalition forces to use airpower in support of the rebels. Operationally, the establishment of air supremacy requires attacks against Libyan air defenses and against Libyan airbases, which are not necessarily part of a no fly zone. In Operation Northern Watch (the no fly zone over northern Iraq) US aircraft normally only fired upon air defenses if those defenses engaged the US planes, and only shot down Iraqi aircraft when they strayed across the line.
Odyssey Dawn is a far more ambitious operation, as has already been indicated by the coalition airstrikes in support of rebel fighters.