FCP's DV NTSC profiles default to interlaced (lower field first), though you can change this by simply right clicking the sequence, going to settings, and setting field order to progressive (or "none", I don't remember the exact dialogue).
From my understanding, this doesn't entirely destroy progressive 30p footage if it's in an interlaced sequence, though chroma handling may be messed up; the real issue is inconsistency in appearance. If you do dissolves in an interlaced timeline with progressive footage, the dissolves (and all transitions/effects for that matter), will appear different in motion, due to the appearance of 60i.
Remember, you must remove all footage from you sequence before changing settings, though a field order change may be possible with footage too.