One other thought since the ADM mass has been mentioned. The Wikipedia article on "Mass in general relativity", here...
http://en.wikipedia.org/wiki/Mass_in_general_relativity
...has the following interesting statement:
"In a way, the ADM energy measures all of the energy contained in spacetime, while the Bondi energy excludes those parts carried off by gravitational waves to infinity."
Wald (1984) is referenced. I have seen statements like this elsewhere as well. Given the definition of ADM mass vs. Bondi mass, this makes sense: ADM mass involves picking a spatial 3-surface out of the spacetime, doing an integral over a 2-sphere in that 3-surface, and taking the limit as the 2-sphere goes to spatial infinity (or, equivalently, as the radius of the 2-sphere goes to infinity). That means that, even if a system is emitting gravitational waves, those waves are still somewhere on any given 3-surface, so they will eventually be contained within the 2-sphere of integration as the radius of the 2-sphere goes to infinity, and hence the energy carried by the waves will be "counted" in the ADM mass. (Since the ADM mass integrand involves the metric coefficients, not the stress-energy tensor components, the wave energy is unproblematically accounted for even though the waves are in vacuum, i.e., zero SET.)
The Bondi mass, on the other hand, evaluates a similar integral at future null infinity, so the gravitational waves will "escape" from the region that is being integrated over, and hence their energy will not be "counted" in the Bondi mass. So in order to determine whether a particular asymptotically flat spacetime is radiating GWs or not, one would compare the ADM mass to the Bondi mass and see if there is a difference.
This also helps clarify what Birkhoff's Theorem is saying: for Schwarzschild spacetime, the ADM mass and Bondi mass are equal, so any spacetime that is isometric to Schwarzschild spacetime outside some finite radius r (which applies to any spherically symmetric spacetime with an exterior vacuum region, by BT) will also have both masses equal, and therefore can't contain any GWs.