I am told a plagal cadence is IV - I and an imperfect cadence is (any chord) - V. So suppose I have the cadence CEG - GBD. This could be I - V, which is an imperfect cadence, but it could also be IV - I (because C is the subdominant of G major) which is a plagal cadence. Does this mean it is both or am I misunderstanding something here?
Depends on what key you're in. If you're in C, it's an imperfect/half cadence. If you're in G, it's a plagal cadence. This ambiguity is a big part of the reason why it's so easy to modulate up a fifth.
Context is everything. If you come to C - G along the lines of C - F - C - G, and following with a phrase starting with C, chances are very good that the phrase will be taken as using a half cadence in C. If, on the other hand, the progression ending in C - G is along the lines of G - D - G - C - G, and the final G lands on a very strong beat, it is probably going to sound plagal. As Matt Putnam states, there is enough potential for ambiguity to enable modulation.
It's probably worth remembering that two chords alone are rarely enough to establish function and tonality.
You actually give the answer yourself....your definition of plagal is correct, as is your definition of imperfect (anything to V). The important thing to remember is that V is not where you want to end up. It is called the dominant chord, and in the context of western music, the dominant chord demands, sooner or later, to be resolved to the tonic chord (the main tonal center of the piece you are playing). In contrast, the plagal cadence (or "Amen" Cadence) leaves you feeling that the piece (usually a liturgical or religious piece, but not always) is in fact over. IN general, chord progressions ending on a V are resting places, requiring further travel through the harmonic landscape, whereas chord progressions ending on I are arrival points, usually signifying the end of the piece, movement, song.