Well... With James I had my show... then 30 mins or so later contractions started... I had him just over 6 hrs later.
With Madeleine, I was well and truly in labour when I got my show... I remember being in the shower, feeling something *there* going to the toilet and noticed the show... it wasn't something I could miss though... I wouldn't have just passed it and not noticed because of its size IYKWIM.
The plug can come away a little at a time too though, just look out for chunkish type mucus... or your waters breaking!!! lol