that exact scenario actually happened to a patient of mine, back before there was such a thing as medical marijuana in AZ. it could've resulted in a dui at that time as well so it was surprising to both of us that he only got charged with possession. the cops were complete dirtbags and treated him like shit though, not sure why they didn't add the dui charge. if he'd had money for a half decent lawyer he probably wouldn't have the conviction now, sucks how the system works...
the dui risk is a good point though as im sure people don't always get so lucky in that regard. especially with the biggest a-hole cops, i imagine they must make a lot more mj-dui arrests these days on mmj users in order to help cope with the disappointment of not being able to ruin their day/life just because of the plant itself. I'd take a dui over a drug possession charge any day, but obviously both suck a lot. the stigma carried by a dui charge, and the way employers look at it, is far less damaging than a drug charge. not to mention it isn't a felony! fuckin ass-backwards AZ is
but as far as paraphernalia goes, i personally wouldn't be worried about any charges directly related to the pipe/bong/whatever when i have my card. I'm no lawyer so don't take this as fact, but im pretty sure the paraphernalia is only illegal if has been used for illegal drugs. so if the only drug residue in it is from marijuana then it's totally legal (again, just my presumption?). but i agree it's good practice to never have anything marijuana-related in your car when you can avoid it, since those items just tip off the cops to the fact that your blood/urine will test positive, and then it's completely up to them whether they want to charge you with a dui or not.
edit - sorry for hijacking the thread this is completely offtopic