I like the idea of my kids having something to look at when they are lying in bed. They had mobiles on their cots when they were tiny but I had to take them when I found my daughter using her newly found skill of sitting up to tug on one of the hanging toys. That could only end in disaster.
It seems to be a done thing that we hang, stick and hammer things onto bedroom walls for children to look at. When thinking about your baby's room or when buying a present for someone.... don't forget the ceiling!