I wouldn't think so. I've accepted multiple sign on bonuses, and I have left before the term was completed many times. The bonus contract should spell out the terms of termination. Some require repayment in full, while others are pro-rated. I don't think it would take an attorney to interpret this- they're usually written pretty clearly. The first two times, I got an attorney involved and they just had me follow what the termination clauses said, and leave. Subsequent times I've done it by myself and been fine.
It is worth mentioning that bonus wages repaid should come off at tax time. Don't forget that.
The only time that this kind of scenario has presented me with problems is when the bonus contract was vague in terms of dollar amounts. One contract stated I would repay $50,000 plus "general expenses incurred for continuing education and license." Upon leaving I repaid the 50k, and the costs of the CE/license, but received a bill from the company for $15,000- the salary of the person they brought in while I was away at CE. I ended up paying it to get them off my back, but I never signed vague contract again.