Is it unethical to accept a job because you need a job but keep interviewing for a better offer?

That would be a matter of opinion. I don't think there are any "official rules" to what's ethical or what's not. I think that if you have some entry-level job, say, at Taco-bell or something like that, they probably expect you to find something better. If we're talking entry-level, blue collar stuff, I don't think it's going to be a big deal. But lets say you are teaching at a university, and quit mid-semester for a research job that pays more - that might be something I would consider "unethical".

Answer

As a former recruiter, I've dealt with this issue firsthand. A person, who I placed in a permanent position, did not disclose to me or my client that he was still interviewing for other positions. After accepting the job with my client, and working a total of 30 days, he submitted his letter of resignation to the manager without any warning.

I was shocked. Do I consider what he did unethical? Absolutely. Because he wasn't forthright with me during the interview process, I had to search for a new candidate to replace him and repair my company's credibility. If he told me the truth, I would have gladly placed him in the position temporarily and negotiated a higher hourly rate to help during his transition to the dream job.

As long as you tell the hiring manager or recruiter upfront, that you are considering other positions, I don't think your ethics will be called into question. Who knows you might even score an awesome freelance assignment with this company because you told the truth.

Answer

This is a personal matter. There are no rules which say you must tell your recruiter - afterall, what good will it do to your credibility if you told your recruiter you were going 'out' with another recruiter, hey? I think the recruiter must have a policy which states that you must finish your minimum term. In this case the worker must have took it as 30 days or there was no policy mentioned.