Was poking around Doyle's Super System 2, and found this little tidbit (subject is playing AA):

If there is an Ace on the flop, there will always be a possibility that your oppenent will have a straight draw (at least).

Low straight draw - A + any card 5 or lower (A49, A27, etc)
High straight draw - A + any card T or higher (AJ6, AT7, etc)
Middle straight draw - A + 2 cards between 5 and T (A79, A86, etc)

I don't know if this is common knowledge or not, but its good info so I thought I'd post it here. Doyle says play your set of Aces fast, don't let someone draw cheaply to a gutshot.