What is a good depth for putting bark around a playset?
The Consumer Product Safety Commission has a chart for this:

http://www.cpsc.gov/cpscpub/pubs/323.html

Fall Height In Feet From Which A Life Threatening
Head Injury Would Not Be Expected

Type of Material            6 "Depth    9 " Depth    12" Depth
Double Shredded Bark Mulch  6           10           11
Wood Chips                  7           10           11
Fine Sand                   5           5            9
Fine Gravel                 6           7            10