A few folks (including us) have iPhone 3Gs units that sometimes say ‘Charging is not supported with this accessory’ - it appears to be a wonky dock connector… jsappo writes -

Hey everybody! I just encountered this same problem and fixed it! It’s a wonky dock connector straight up. I have outlined the entire process for fixing it.. Hit me up with any questions.

Makes sense, the 3Gs units are the only ones that have given us any problems.