First of all, it is likely a violation of PayPal's Acceptable Use Policy as of Oct 2011:

3(h) involve currency exchanges or check cashing businesses

Given this, it's unlikely that PayPal would be amenable to helping at best, and may close your account (and freeze the funds it had for six months) at worst.

Another chief problem is the high likelihood of chargebacks. PayPal has generally sided with the buyer when any dispute is brought against the seller for any reason. For this reason alone, it's best to accept Bitcoin only when you trust the buyer and have some influence on their reputation.

Furthermore, Paypal has high fees (current 2.9% + $0.30 USD).