I believe my pumpkin plants have recently developed powdery mildew. My question is should I be picking these leaves off my vine or because harvest is almost here I should just leave it alone? Also if I were to pick these leaves off the vine are they OK to put in my compost pile or would that spread the mildew into future plantings?

I have the same problem on zucchini and cucumber. I removed the cucumber plant altogether because it had finished. I might remove the worst leaves on the zucchini but for sure will pull the plants when they are finished. This Extension article says don't compost them:

http://articles.extension.org/...

I don't grow pumpkin but I grow zucchini and summer squashes. All in the same family with the same mildew problems. Usually I just leave the mildew as I have found once it starts it progresses very rapidly and no use fighting it. Once it gets too bad and any fruits have been picked, I just pull the plants. I always bag them for the yard waste pickup as I never want to compost squash what with the problems between those mildewy leaves the the Squash Vine Borers.

I agree with both Sue and Rita; if they start getting mildew earlier in the summer I'll usually spray with a fungicide, but this time of year things are getting pretty close to done here and I just let them go and harvest the pumpkins and squash once they look mature enough.