This is probable by design and a good feature. I sync BaseCamp data with 3 Macs using my free google dropbox folder. In the past, I just started BaseCamp on each computer and got the new updates when they were released. With version 4.0.0.8, after the update on the first Mac, BaseCamp would not open on the other two computers. I got a continuous bouncing BC icon on the MBP and a crash on the iMac. I'm running Mtn. Lion on all three Macs. I have been worried in the past about opening my updated data folder with a previous version of BC so I am not complaining. I think it would be better if I got a dialog box saying something like; your database has be updated to an later version of BC, lease update BC.
That is what should have happened, but we really haven't done any testing of your case of sharing a single database between 3 computers. It's not that surprising there will be some hiccups.
