Big news: Jack has spent four full nights in his own crib! Many thanks to Greg's sister and her husband (Jen and Pete!) for passing down this crib, and of course to Greg's parents for the original gift.
Moving Jack from our bed to the crib was much easier than I thought... we now have a baby who will asleep within a minute of being put down (AMAZING). His sleeping pattern seems to be the same as when he's in bed with us: he's mostly up every 2-3 hours, but there's an occasional 5 hour stretch in there sometimes.
I'm reluctant to night-wean Jack (to stop breastfeeding him at night) because we are having trouble nursing during the day (he's so distractible than he just can't seem to nurse for more than 20 seconds at a time). I think if I night weaned him, he'd probably sleep more... but I'm just not willing to risk decreasing his milk intake by that much. For now, things are working.