There's no reason a potential employer should know about a medical condition unless a) you told them, b) you told someone other than a health care professional who told them, or c) it is physically evident.

The reasons in favor of disclosure include accommodation and protection under the ADA (Americans with Disabilities Act, I'm assuming you're in the US based on spelling!) But as folks here will tell you, there's no guarantee of justice and sometimes people get sacked when their condition becomes known. The earliest to bring it up would be during negotiation after you've been offered the job. Or maybe never. Disclosure to an employer is a very situational thing.

I'm in a similar boat - going to school and worrying about how a bipolar dx will affect my job prospects. But the actual issue is more with the career than a job; I can get a job and my dx doesn't have to influence that. Making sure it's a job I can handle is a much bigger problem, since stress is a trigger.

One caveat is that an official bipolar dx may haunt your insurance records - which is a different issue than employment. Psych professionals are aware of this and may provide a different dx for paperwork purposes. My ICD-9 dx code is 296.90 - "Unspecified episodic mood disorder" - which apparently doesn't raise the same flags with the insurance.

Definitely look up HIPPA and go talk to a professional. The only times they can disclose anything without your consent is if you are a danger to yourself or others. Take care of yourself!