The isotope ratios within the different planets are sufficiently distinctive from each other, but there are other bodies that can fall on the same line with the same isotope ratios, here's an example graph of Oxygen isotope ratios: http://www.saharamet.com/meteorite/gallery/cut872.html (see graph 2)
For example, the Enstatite Chondrites (E-Chondrites) have Oxygen isotope ratios very similar to those of the Earth and Moon, but this doesn't mean that they came from either of those bodies or that the impactor that made the Moon was an E-Chondrite. It just means that either they came from the same chemical resevoir or from a chemical resevoir similar to that from which the Earth formed.
It is sufficient evidence for the general public, but not meteoriticists. Scientists strive for 100% certainty, but sometimes have to settle for 95%, most of the time much less...
If you take a class on meteorites you would probably learn far more than you want to know ;)
Mike Gerszewski Univ. of North Dakota Space Studies Graduate Assistant