can you share a calendar without MobilMe?

My co-worker and I started our own company and wanted to use iCal for our business. I have MobilMe and he had a trial basis of MM. We had our work calendar on both our macs and they sync'd fine (He'd add an event and I would see it or vice versa). His trial version ended and he decided he did not want MobilMe. Now a calendar won't sync up anymore.
Does he have to have MobilMe for us to use iCal as a shared work calendar that we both can access and make changes to? If not, then what are we doing wrong?

Thanks, Jeff