Johnman, if the APU (bleed) fails during take off, the eng bleed configuration will not change from closed until the thrust levers are moved to CLB. The Bleed warning is inhibited from T/O thrust selection until 1500ft so the only warning you will have in the cockpit getting quieter and warmer...So you can do a take off with the bleeds on APU without fear of losing performance should it fail. At least that is what I understand to be the case, I don't have the manuals on hand so no reference sorry...