You ask a very interesting question.
Is Venus spinning "backwards?"
Or is it "upside-down?"
It's very slow, retrograde spin makes it very unique in our solar system. Technically, Uranus is titled to it's "south pole" is slightly inclined to the northern side of the ecliptic as well, but it is so close to 90 degrees, that it seems almost negligible.
Still, something is different about both of these planets.
So, what is it that causes this?
Impact?
Gravitational Influences?
"Captured" objects?
Tidal forces acting peculiaraly in the past?
"Friction" with it's atmosphere?
Initial formation conditions?
Well, no one knows for sure, and there are many hypotheses out there, but some we can rule out pretty easily.
Truth is, I'm not sure which one explains it best, but here are some very good guesses:
http://physicsworld.com/cws/article/news/2661
http://www.astronomycafe.net/qadir/q50.html
http://ftp.astronomy.ohio-state.edu/~pogge/Ast161/Unit6/venus.html (scroll down to "Retrograde motion, possible causes)
But I think the most accepted, as of now, answers can be found here:
http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/981026a.html although it is worded poorly. Basically, this link states that it was *probably* due to initial formation conditions, OR there was a major impact.
