There was some air-to-air combat in the Gulf War and some Allied planes (including CAPT Scott Speicher's I believe) were lost to enemy aircraft fire, none so far that I'm aware of in the war on terror (in either of the major campaigns or smaller actions in the Phillipines, Georgia, and Djibouti). The Ba'athists in Iraq had an air force but its pilots refused to take off (or may have never been ordered to do so in the first place) knowing that they would be shot out of the sky the minute they did so, and the Taliban didn't have an air force at all (a few rickety, 30-year-old Soviet helos I believe and that's about it).
Only a few countries have enough air power to even engage the US Naval aviation/Air Force in the skies (UK, France, Germany, Russia, PRC, ROC, the Koreas, Israel)--by that I mean they would be capable of engaging the US in a fight and one of their planes might possibly manage to shoot down a US aircraft before being itself shot down. And of these, obviously North Korea and China are the only ones there's any chance we'll be at war with anytime soon.