Most campuses call this "challenging" - I did it several times. In city college, I successfully took courses without their prerequisites by speaking with the professors and showing them my work/experience. It wasn't test-based, just portfolio review.
In university, successfully challenging also gives you credits for that course (though there is a small fee per unit). I successfully challenged four courses (all in graphic design) - three of which were based on portfolio review and the fourth was based on completion of course projects in an accelerated period. Additionally, though not quite a challenge, I was able to speak with a couple CS professors and get them into admitting me into their courses despite not having the prerequisites (though was doing them concurrently).
The course requisite structure is more a guideline than a rigid computer system that works one way and only that way.
Berkeley used to let students skip the introductory CS course, 61a (in Scheme), if they had received a 5 on the AP CS exam. They removed that option I believe after fall 2010.
One of my friends was able to skip 61a by meeting with the department chair, Paul Hilfinger, and convincing him that 61a would be a waste of time for him. My friend could drone on and on about the simplest of subjects so we assumed he just annoyed Hilfinger until he said "enough already".
My university did. COS101 was an optional class for complete novices but if you weren't already at least exposed to programming your advisor would sign you up for it. COS125 was the first for-realsies class (taught in Scheme, as it happens).
