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The diagnostic test for whether it's the plastic vent valve gizmo up near the filler pipe door is to depress the little tab on the edge of the filler opening ring. I don't have my car right now so I can't take a picture but it's the part that the edge of the gas cap depresses when you screw it down. The little tab moves down about half an inch and moves the valve into a position where you can get gas into the tank even if the magnetic switch is not working. This isn't how the valve works in regular use but it's a work around for a tank you can't fill. You'll need a small screwdriver to press down in the same spot the edge of the gas cap would normally hit.
If you can get it to take gas when you depress the tab but it won't take gas otherwise it might be the magnetic switch (assuming the fuse is good). On my car the magnetic switch wasn't where it needed to be because I worked on the vent valve and didn't put it back correctly. A new vent valve comes with a new magnetic switch. It doesn't cost very much and it's not too hard to replace once you've removed the right front fender liner. The plastic valve just clamps around the filler tube and one bolt holds it in place at the fuel door. You have pretty much total access to the charcoal cannister, the tubes that connect the venting system to the cannister and the valve block behind the right side headlight. That valve block has two small electricly operated valves. One is spring loaded open and the other is spring loaded closed. The car's computer cycles those valves in various ways when it does an emissions check on the fuel tank and during normal driving. The Durametric can cycle the valves for testing and you can hear them clicking.
I'm not an expert on Boxsters - I'm just telling you what I found out when I worked on my car. Your car might have a different problem.
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