I have a client experiencing problems using the same account on multiple computers. Here are the details:

Two people are sharing the same credentials using the iSync connector so they could modify the same calendar correctly. We had issues with permissions on shared calendars...

The same two people login with the same credentials using Active Sync on the iPhone. They are using the same account so either one of them could modify the calendar over the air. This is also because shared calendars do not work with the iPhone over the air.

When one of them deletes an event (either over the web interface, iCal or iPhone), it re-appears after an hour or so. This is causing big scheduling problems with them.

They're running OS X 10.5.5 with iSync Connector 5.0.2672