Should I be concerned that my baby is not babbling at 6 months?

No. I wouldn't necesarially be concerned at this age if he or she was cooing at 4 mo. To learn more about speech development read: http://sparkspeds.Blogspot.Com/2011/07/major-speech-milestones.Html.
Yes. At six months, most babies are very "vocal", making vowel and consonant sounds, as well as squealing and "razzing" sounds. They tend to be be very social, enjoy vocal interactions and music. If you have concerns your baby is not "exploring" his/her voice at this age, check with your pediatrician.