The United States has had almost unchallenged air superiority in every conflict it has fought since World War II, and the last time that a U.S. soldier died as a result of an enemy airstrike was 1953. This was largely thanks to the F-15, which has been in service since 1972 and which the Air Force plans to keep going until 2025. The putative replacement is the F-22, but Congress only ordered enough F-22s to replace a third of the F-15 fleet.
The F-22 is the most expensive fighter ever built, and it has fallen victim to the vicissitudes of post-Cold War military funding. It was initially planned as long ago as the 1980s, when the main military threat facing America was a Soviet Union bristling with fighter jets. The Osama bin Ladens and Saddam Husseins of this world tend not to put up much of an aerial fight, and military spending has been adapted to focus on more contemporary threats.
However, the rest of the world has been busy closing the technological gap, which means that in the future the U.S. will be less able to control the skies. This has raised fears in the defence establishment that if the military is called upon to fight "the Big One" - a huge conventional conflict with Russia or China - it will be less able than it might have been; vulnerabilities also make conflict more likely.
F-22s take as long as three years to build, so it will not be easy to produce them quickly if the need arises. The consequences of these sorts of procurement decisions take decades to reveal themselves, but the centrality of air superiority in every conflict the U.S. has fought in the last sixty years ought to give pause for thought over this one.
BrevityQuest09. This article is an excellent discussion of the programme, whereas Thomas P. Barnett's The Pentagon's New Map discusses trade-offs in military procurement.