Hmmm...no problems immediately recognizable. Which modem are you using? Is it a "hardware modem", a winmodem, or a softmodem? I've had trouble pinging with certain "hardware modems", although they perform better overall.
Try pinging something by IP address: 126.96.36.199 (it's a domain name server).
Also, make sure that in KPPP you have it set to automatically make it your default gateway.
Hope that helps. Anybody else have an idea?