I am about to start a PhD (Oct 04) in Biochemistry with two supervisors and I have recently found out one of them is leaving to for a job abroad (Jan 05) without even telling me. I am concerned that this will effect my project as there will be nobody there to help me with his side of the project, and also with his casual attitude about going without saying anything to me. Do people think this is a bad omen for the future?

I don't think this is necessarily a bad omen. With PhDs taking 3-4 years to complete, it is not that uncommon for supervisors to change institutions, or go on sabbatical abroad, during a students registration period. Your university is obliged to provide you with adequate supervision and should appoint another supervisor to guide you if your remaining supervisor cannot provide expertise in a particular area.