Let's try a pragmatic compromise (i.e. everyone will be unhappy, but it's already a fait accompli).
In SVN 2000 (yay me!) on the trunk, position2d is now a typedef/#define of vector2d. position2d is very tentatively marked as deprecated. You can continue to use it, but you'd be as well using a vector2d, since that's what it is.
I don't think that we can or should get rid of dimension2d since it would stuff up apps that rely on Width / Height rather than X / Y. However, I have added more implicit conversions between dimension2d and vector2d.
You can probably do some real damage with these, by accidentally passing the wrong types around without realising it. Knock yourself out, and please let us know if this is going too far, or not far enough.