I store my family tree database to my iDisk (MobileMe). However, sometimes MacFamilyTree will hang when I save my changes.
The performance of your iDisk depends on a lot of different factors, including your internet connection's upload speed. MacFamilyTree does offer publishing of your family tree as a website to MobileMe. Saving your family tree database to an iDisk is not officially supported. The iDisk unfortunately doesn't behave like a local folder.
If you're looking for an alternative, maybe consider DropBox. The DropBox exchange folder behaves very much like a local folder and several customers reported success in working with it as their storage location. Please keep in mind, though, that none of these options are officially supported since their behavior can change at any given time without us noticing.
More general information is available in the User Guide. You can find it in MacFamilyTree's Help Menu or for download here.