Ok I did some reading and I think this is what you are after:
mpeg2source("palclip.d2v")
bob(height=480)
bicubicresize(720,480)
changefps(60000,1001)
separatefields()
selectevery(4,0,3) #use selectevery(4,1,2) for odd field first.
Weave()
This is just copied from the wiki.
EDIT: Just tested this and it seems to do exactly what you want. I fed it a pal clip and out popped a 30i clip with the exact same playing time.