The spell description specifically states that it is immobile, but if you cast it on the back of a wagon, I would rule that it stays on the wagon. After all, the wagon is moving. The force cage is not.
I'd rule it like an immovable rod. It is also immobile, and can not be moved except by extreme strength. So I'd say that the cage, won't move with the wagon, but will simply stay in mid air if the wagon leaves it.
I'd go with Stalker0 on this one. Even if something is moving because its being carried, it is still moving. The spell description explicitly states it is immobile.
I've been thinking about this, since I have an opposed view on each post . . .
In the end, things like this come down to the DM's judgement...
For me the diffrence is that a portal is 'anchored' in my mind, it works in refrence to some doorway or such. So in a floating castle it's in refrence to that castle. A forcecage in such a situation may very well crash through walls...
Remind me to build my cosmoligy with the world imobile at the center of the prime material plane and the heavens set into crystal spheres rotating around it, so I don't need to be bothered to bring reletive motion of planets and solar systems into things...