If a recruiter contacts you and inquires about your current employment status, does one have an advantage in saying that he is currently employed as opposed to unemployed? Moreover, does the recruiter expect you to have a job in the field for which he/she is recruiting? Does it really matter at all?
Do you have an advantage if you're currently employed? Generally, yes. Employers generally prefer hiring people that are currently employed and those that are currently employed generally have more leverage in negotiations.
Does the recruiter expect that you have a job in the field? If you're employed, generally yes. It is possible, of course, that the best candidate for a job is someone that is switching fields. But it is generally much harder to place a candidate that is changing fields since it is often hard for an employer to understand exactly what skills transfer from one field to another.
does one have an advantage in saying that he is currently employed as opposed to unemployed?
Often, the answer is Yes.
Being employed means (by definition) that you are employable. It means that you passed the interviews for some other company, and thus might have a better chance of passing upcoming interviews.
Rightly or wrongly, many recruiters and hiring managers value being employed - either consciously or unconsciously.
Some recruiters and hiring managers (fortunately not all), believe that the best workers are seldom unemployed, and are very seldom unemployed for a long time.
Moreover, does the recruiter expect you to have a job in the field for which he/she is recruiting?
That almost always depends on the specifics and requirements of the job.
An entry-level job by definition wouldn't expect a candidate to have a job in the field.
Some jobs can be filled by candidates who don't work in the specific field.
Others (for example, more senior positions) expect current job experience in a particular field.
Does it really matter at all?
Everything matters. Sometimes it's critical. Other times not as much.