Before MobileMe debuted, we had to set up a Mac-using executive and his two Windows Outlook-using assistants with calendar sharing. The exeucutive already sync'd his iPhone calendar to his Macbook Pro. So we set up a Google calendar. We installed Spanning Sync on his Mac (and in addition to the other comments about what you get for the price, you can use one Spanning Sync license to sync multiple Macs to the same Google calendar). We installed a Windows app called SyncMyCal on the PCs. Now all three can update their personal calendars, sync certain calendar or all data to Google, and use Google as a bridge between them. The upside is, Google can also be reached from other Macs or PCs that are not part of this setup.