According to some groups, there might be an increased risk of SIDS or suffocation on pillows/blankets (particularly for newborns), but there are also quite a few pediatricians who believe there are major benefits in co-sleeping. It seems to be unanimous that sleeping in the same room as the infant is highly preferable to having them in a separate area of the home, however.
Ultimately, it comes down to what the parents feel is best for them and their child.