OK, so....
abge is right that the spending clause is pretty broad. The federal government can spend money on just about anything it wants that doesn't violate some other explicit principle (it couldn't set up a church, for example).
Krellin is right that the powers are enumerated. One of the powers is spending lots of money on all kinds of things. But that doesn't mean the powers are unlimited. For a notable example, the federal government can't pass a law saying you can't have a gun within 1000 feet of a school. You'll notice that when it tried to do that (and it did), it wasn't spending anything. It was making an action illegal, which is completely different from spending, and which it can't do. It would probably also be disallowed from a general anti-murder statute, or many, many other things that government does that have nothing to do with spending.
There's a really fascinating discussion of this point in Joseph Story's commentaries, for anybody who cares to look it up.
Oh and also, the third day of recent health care arguments focused on a really interesting and very related point that received a lot less media attention than the mandate. The argument was that if the federal government gives lots of money to the states, with lots of very specific strings attached, then it's actually effectively usurping state autonomy, so even though what it's doing appears to be mere Constitutional spending, at some point it crosses over a line and becomes unconstitutional meddling. Interesting stuff. You should check it out.
Finally, to those who are whining about the Constitution being outdated -- there's an amendment procedure. If there's really a need to change something because of changing times, then the Constitution could be amended. Most often, people who say something about how the Constitution can't be treated literally because it's "outdated" and you can't run something like that from the 18th century actually want something that they would NOT have the votes to get through as an amendment. Ask yourself, if you believe in democracy or the rule of law, if that's not a little troubling. Is the text really "outdated"? Or is it just inconvenient to a particular political outlook that doesn't want to have to screw around with elections to get stuff done?
Abge, as to your air force example, it's going to be hard to get anybody to take it too seriously, legally. Even if you found such an uber-textualist that he wouldn't take the small leap of interpretation to say that it was implied by the text, it would still be true that if he did actually rule it unconstitutional, there would be an amendment TOMORROW making it Constitutional again -- how the process is actually supposed to work when an "update" is needed, by the way. In the event, I doubt it's ever actually been litigated (though I have not looked this up).