resign anytime?

If you signed a contract, there should be specific language in the contract specifying what your obligations/penalties are if you fail to fulfill the requirements of the contract (i.e., resign before the contract is up).

That all depends on the terms of the contract that you have signed. It would be wise to re-read your contract because, in all likelihood, the answer to your original question lies somewhere between the fine lines.

It would be unwise to break any agreement, unless the facility you are working for is not fulfilling their end of the contract. I don't know your particular situation, but you don't mention any breech of contract on the facilities part. If you believe your contract facility is in breech of the contract you should ask them to live up to the terms of their agreement. If they fail to do so, you may be justified in giving notice and resigning for breech of contract. If on the other hand you are simply shopping on your own, you may have an ethics problem.

I agree with wjf00, it sounds like you are breaking the terms of your contract because you received a better offer. You may have an ethics problem, but you are risking your reputation. Nursing is a very small community, managers and directors talk to each other.