Hi! I am a German citizen studying in the US. I have an US MS degree already, and I am thinking about applying for a job in the US. Can you switch from a F-1 student visa status to an H-1 visa status? I assume I'd have to have a job offer first!? What would be the procedure? What US agency would be responsible for that (or would that be my university's ISS office!? )? THANKS MUCH!

Hi! I can tell you how it worked for me.
Every F1 visa includes one year of OPT (Optional Practical Training). You can use the time while you study (for instance during the summer) or after you finish your degree in the US. You apply for the OPT permit through the Office for International Students at your University. Most people use the year to find a company that wants to hire them and is willing to sponsor them for the H1b.
When you find an employer who is willing to sponsor you, they (with a lawyer) take care of the paperwork. There are several steps, they take 1-2 months, and when the application is complete it is sent to the INS. The INS has a quota of H1b visas for each calendar year. They start 'giving them out' in January and sometimes they can run out of the visas earlier than December. When I applied they ran out in October and my visa was one of the last ones they approved. So it is important to start the process early in the year. If your job is in academia (as a university professor), there is no limit to the number of visas they approve.
H1b visas are for 3 years and can be extended (for an additional fee) for another 3 years. After that the company has to apply for a new H1b visa for you. Usually companies apply for a green card for you (if they want to keep you) as soon as your H1b has been approved. Last I heard green cards took approximately 4 years to get approved.
You can NOT apply for an H1b visa on your own. If you go to recruiting agencies to find a job, make sure they understand your status. Many will tell you they do NOT accept job seekers unless they have a green card (permanent working permit). If you have any contacts, it's probably best to try and get interviews without going trough recruiters. This doesn't apply to IT (computer science) jobs, as they are (or used to be) in very high demand. Certain science/health type jobs may be in the same category - a lot of demand for qualified people and not enough US candidates.

Once your H1b is approved you have to go to an US consulate to get your F1 changed to H1b. There are no US consulates INSIDE the US, so you'd have to go to another country. Canada was easiest for me - you make an appointment online at the respective US consulate (many weeks in advance), they send you a letter which you bring with you. Don't forget you may need a visa to enter that country as well - probably a tourist visa. More detailed info about appointments is available on-line. Good luck with your job hunting!