I'm a huge advocate of getting moms to let their babies play on their tummies when awake, simply because I have learned from my early childhood classes that though SLEEPING on the back is VERY important in cutting the risk of SIDS, spending time on their tummies when awake is terribly important for muscle, spine, head, and brain development. When babies don't get enough tummy time, their muscles remain weak, making it harder for them to transition to sitting up, crawling, and later walking.
Most of all, lack of tummy time can affect the development of the skull, which in turn affects the space in which the babies' brain is growing. Also, severe lack of tummy time can cause the head to develop with obvious deformities in shape, such has completely flat heads, heads that are off center due to weak neck muscles, and cause other areas of the head to protrude at odd angles. All can be fixed by having your baby wear a "helmet", which a doctor can tell you about or recommend, but really, if it is avoidable, we as parents might as well avoid it.
Well, let me just say I was a little excited to see that our local news had a short info segment about the importance of tummy time (thus prompting this blog entry). You can find the link HERE along with a video of the segment and the information the doctors have to share.