You are making the assumption that an all-powerful being is exempt from all rules.
But if we're speaking rationally, a creature with infinite strength (as only one example), would still have to obey the laws of physics in applying that strength. His strength does not allow him to break out of those laws.
On the same note, an infinitely powerful being would still be bound by physical realities—he would just be the master at manipulating them. This could preempt any paradox like that which you bring up.
But this is more to the point: If God is perfect, and perfectly honest, then he is inherently restrained, for if he says anything, he is automatically bound to keep his word, and cannot act in a way that would break his word. You might consider that a limitation of his power, but I would consider it the fulfillment of his honesty.
So, you have to tell me: Why would a creature create an object they couldn't move with the intention of moving it later on? (Remember, it is assumed that God is also omniscient.)
Kind of makes your paradox theoretically impossible, I fear.
Edit: But more to the point, you are simply arguing that omnipotence is impossible. First you define omnipotence as having the ability to do all things, regardless of their purpose, necessity, or accordance with universal principles. Then you say that it is possible for someone onnipotent to create impossibility. But by your very definition of omnipotence, you have rendered this impossible. Think about it this way: If God were to strip himself of omnipotence, he would no longer be omnipotent. On the same note, if God were to make something impossible for himself, he would simply be stripping himself of his own omnipotence. This does not mean that he was not omnipotent to begin with, simply that he has the ability to limit his own omnipotence in the future.
An entirely separate logical escape from your paradox is this: If God created an immovable object, God can also, presumably, remove the immovability from that object, and make it moveable again.
(Using standard physics models, an object so heavy it cannot be lifted by something omnipotent is a physical impossibility, as weight is limited by gravity, and anything too "heavy" [i.e., dense] becomes weightless within the context of space, while force has no such limitations.)
So, there you have it: 4 solutions to your problem:
1. An omniscient creature could not fathomly do such a thing.
2. Omnipotence need not be eternal, and as such your scenario wouldn't nullify its existence.
3. Limitations on omnipotence could be interepreted as being necessarily inferior to the omnipotence itself, invalidating suggestions that omnipotence could be limited.
4. The scenario you give is a physical impossibility, and such a scenario is absolutely incomprehensible physically.
