open cans are just the wrong thing for public use, especially in an office setting. Sooner or later, you will bother somebody. A lot.
A rule of thumb: if there is enough ambient noise that someone isn't able to hear open headphones at six feet, then there is also enough to ruin the quality of the sound you're hearing. Nobody will hear open 'phones next to a highway, not even if they're wearing them.
Of course, different open 'phones leak different amounts of noise, like your little PX100's, say, so the distance can slide up or down a bit depending.
With all due respect to F107plus5, I once wore my Grados in a quiet office setting at a distance of roughly 18 FEET from the only other person there, and within ten minutes he was tapping on my shoulder with an angry look in his eye, begging me to turn my ******* music down. That was the last appearance my Grados ever made at the office.
So Vosper, I'll recommend that you take the time-honored head-fi route: you shouldn't have just one pair if you're going to use them in different places or for radically different kinds of music. That is, just keep the 580's at home, and you've got the PX100 to tide you over at work until you can get some nice sealed cans.