Well... about a few years ago I implemented a similar system using oscommerce and postnuke.
You can view the site here: www (dot) advantage (dot) coop
....guess I'm not allowed to post urls yet..hmm..
This really isn't a combined system though, just oscommerce and postnuke with the same theme. The mysql is two seperate databases, with the oscommerce one using the postnuke user table. Works fairly well though..
There is a integrated system called OSC2NUKE. I have not tried it but see if thats what you are looking for. Its based on PHP-Nuke and OScommerce but due to lots of bugs with PHP-Nuke I had been reluctant to try that.