Well I don't know if a polar shift and a polar tilt refer to the same thing, but I know I just saw a program on NatGeo the explained the current tilt of the axis(23 degrees or so) was caused when a proto-planet smacked into the proto-earth about 3.5-4 billion years ago. Thus forming the moon and causing other things that helped formed the current Earth. Granted I don't know how old the program was, so the theories may of changed by now.
The hypothesis you're referring to is the collision of Theia (a Mars-like impactor) that occurred 4.6 billion years ago (so right at the beginning), aka the Giant Impact Hypothesis. It's the most widely accepted hypothesis of the formation of the Moon (for various reasons (e.g. oxygen isotopes, plane of orbit, etc.)).
I can't remember going into much detail with what the collision did to the Earth's axis... but it's hard to imagine a big impact like that having no effect on the Earth's tilt and rotational spin.
The moon is also the reason our axis is so stable, unlike Mars whose axis is always varying greatly.
Edited by Unknown Entity, 27 July 2009 - 04:03 AM.