[bobhalford]

Seems like this one is read-dependant. In this case, the sb probably has a Q and bets it because he's afraid of it getting checked through and doesn't want the K or A of diamonds to get a free look at the river. Seems like a good fold. The other play is to call the turn and see what SB does on the river if the straight and flush draws don't appear. He could have something like KJ with a diamond or J9, but both of those are less likely considering you have two jacks.