Our baby sleeps in a sleepsack (the leopard pattern!), but we found that he still needed to be swaddled so he didn't jerk his arms and wake himself up. The solution we found that has worked well is putting one of my husband's long-sleeved shirts under the baby and wrapping the arms around the baby. This way we get the best of both worlds in terms of swaddling our baby and using a sleepsack instead of blankets.
(submitted by Alix, Tacoma, WA)