You need to talk to the HR. Based on the current accounitng job market, it is very likely that Deloitte will apply for H-1B before you start. The earliest date to file application is 6 months before starting date. However, you normally cannot file until April 1st. BUT, if you are a graduate student, you should talk to the HR right now as Bush might sign the new bill into law very soon. The new law will grant 20,000 extra H-1B to foreigners with advanced degrees. Normally the law will take effect 90 days after Bush signs it. So you might be able to apply even earlier than April 1st.
Pls disregard the other answer to your question. There is no probation period for accounting firms, especially Ernst & Young and Deloitte. And Deloitte has its HR specialists dealing with international students. They will understand what OPT and H-1B mean.
Good luck!