In the case where a substantial amount of money may be paid to train you or ramp you up, you may wish to ponder the idea of notifying your employers before you take the job. You may even want to do that by asking directly if there is a sign-on obligation (for example, a commitment to stay for a year after training). Not every position requires a lot of training, or this type of commitment.
For the record, from a management perspective, it takes about 6 months to ramp up just about anybody - in the 0-6 range, the person is more of a liability than an asset as they are (rightfully) asking for help and instructions from the team, so the overall team is less efficient and the new guy isn't productive enough to make up for the overhead of question-answering. At 6-12 months, the ratio levels out - the new guy is more produtive and asks less questions (cause he has the clue!) and he's doing meaningfull work. He is probably not 100% efficient yet, but the team as a whole is now more productive than they were before the new guy joined. The new guy usually amortizes on the investment in his second year - somewhere between 18 and 24 months into the situation.
If you are really sure you're relocating, give your employer a heads up. While you probably won't destroy your career (the world is a big place, and blacklisting is nearly impossible), it's ethical to let them know.
I agree with @atconway that multiple career jumps in a full-time employee is a danger sign. One jump, because of a relocation - is not a killer - but in terms of burning bridges, I think you need to be fair to your incipient employeer on your short term plans.
The metric for "normal" career changes these days is usually 3 years.