I have only Office 2007 installed along with A2003. So except for Access, all other applications are only O2007. I don't have any production databases running in A2007 since none of my clients has made the jump yet. I do most of my development in A2003 but I test in A2007 to be sure there will not be a problem and if there is something I know is easier to do in A2007, I switch. One thing I have noticed is that occassionally, opening a corrupt A2003 form in A2007 fixes the corruption. The big annoyance with using the two versions is the time delay when swapping between them. I don't have the nerve to have both open at one time.
WARNING - if you are using Vista, set up two desktop shortcuts to start Access. One for each version. Set the shortcuts to run with admin permission. Before I discovered this, I had three weeks of hell where I thought I was loosing my mind. You have no idea how bizarre the behaviour of A2003 is when it is running with the A2007 library!!! This is what happens with Vista since unless the program is running as administrator, whichever version has control of the registry keeps it. And in my case I had installed A2007 first so it was in charge and there was no dislodging it.