Cool, so that's how it's done! I was just looking to see where it might make sense to put this, and found Mathematics.Calculator, which does have some methods (VectorToRadians and RadiansToVector) that do pretty much the same thing. Two differences are:
1. my equivalent of VectorToRadians returns null if you give it Vector2.Zero (whereas the Calculator one will return 0 radians--not always the best thing to do, but probably fine).
2. the Calculator ones, when used with a downward-increasing y-axis (like with graphics) treat polar coordinates as going CW (just a case of a sign change).
It's debatable as to which is more desirable in the general case. I'll let you (or anyone who wishes to comment) decide if I should submit a patch for this. I'm leaning towards a no.
I'm going to post another bit of code in another thread... it's maybe a little long to do that, but I'd like to hear if people think it is desirable enough to include in farseer, and it might also be nice to leave it public for people to find through google.