Here's an unlikely post from TV Guide, where Padalecki indicates his thoughts on the whole "has Sam slept with Ruby" issue. I'll just quote the whole thing, shall I?
"Supernatural fans want to know: Did Sam hook up with his demon helper, Ruby? While series creator Eric Kripke thus far is mum on the subject, Jared Padalecki is willing to offer his own (albeit unconfirmed) opinion on the possibly sticky sitch.
Padalecki theorizes that the unlikely associates "have been lovers," noting that "from the first episode this season, when Dean returns and Ruby (now played by Genevieve Cortese) is in her underwear and Sam is coming out of the bathroom, it's hinted that Sam and Ruby have a bit more going on than, 'Hey, let's study how to kill demons.'"
Padalecki, however, stops short of suggesting that any hook-up (if one happened) means anything. "Maybe she's just being nice to Sammy," he says, "since he thought he lost his brother."( Rant, rant, rant! )