I did it during one of my electives way back in the day. Just worked about 2 - 3 shifts with this one attending. Honestly he probably didn't know too much about me, but I was in the same situation. It was in August and most of the big wigs were out on vacation, so it was just the regular faculty. Asked him for a letter, he said yes, and, well, matched in my #1. So, just do it! Its not like you have mcuh of a choice anyways.
Just make sure to phrase it "Do you feel comfortable writing me a STRONG letter of recommendation?" If they say yes, go for it! If they hesitate, say, "oh crap my pager went off, gotta run!"
Q