Chiming in with the same issue. Although Handcent works, delivery confirmation reports are reporting invalid. Have a few things to add to this:
1. When sending from the stock application, then going over to handcent, it is indeed a green check mark, so it's specific to handcent.
2. When receiving a text message, if handcent is installed, the verizon sender WILL NOT get a delivery confirmation. Once uninstalled the sender will get a confirmation. Either way messages still send and receive properly even though the status' show as failed.
So, seems to be pretty clearly a software issue on Handcent's side, but not a deal breaker for anyone that doesn't use that feature. I've turned off the carrier reports for now until the next update comes out.