Background information: poblano peppers are one of the most common peppers in Mexican food. Pasilla peppers are something entirely different - the dried form of the chilaca.  I'm originally from Texas, where poblanos are commonly found in grocery stores. I moved to California a while back, and here they're still pretty easy to find, but they're always labeled "pasilla". Even at a local produce market with a decent amount of Mexican food (and Spanish-speaking employees) - and just recently I noticed they even have anchos (dried poblanos) labeled as "dried pasillas". I did manage to find one local Mexican restaurant menu online describing a chile relleno as a stuffed poblano, but otherwise it seems pretty universal.  So where did this come from? Via the above-linked wikipedia article, I did find a website saying it's the fault of the California produce industry for labeling them this way, but that's not the original reason. Is it all just the result of a single person getting it wrong a long time ago, and an entire produce industry building off of that? 
We use 100's of "pasilla" chiles at work each week (in California).  Although they are all labeled as pasilla, it is obvious that they are poblano.  The poblano, as previously stated, is a dark green, wide shouldered, about 4-5" in length, medium to hot chile.  Also correctly stated earlier, an ancho ("ancho" actually translates to "wide") is a dried poblano.  Pasilla's are actually a fairly difficult chile to obtain on a regular basis.  I wanted to note, however; that the poblano's that we receive are more often than not grown IN MEXICO AND LABELED AS PASILLA.  Curious.  Pasilla chiles are a bit narrower at the shoulders, a bit longer than an ancho and depending on the growing environment, are about the same in heat index.  Most pasilla that are commercially grown are used for dried chile. 