This is all too true. Dropbox is great in that it keeps the versions for the last 30 days (or forever as a paid upgrade). In the event of a conflict, both versions are kept. They also have update tracking which makes it easy to see what machine made what changes and when.Unfortunately, syncing operates under the age-old computer principle of Garbage In Garbage Out. When syncing is working properly, a change made on one device is propagated to the web and to all other devices. If that change was unintended, the service might have a previous revision, but don’t count on it. A problem with data on one device is copied to all other devices in your sync scheme.
These same concerns also hold true for your calendars and contacts from MobileMe. Even though you sync them to the cloud, you should still perform a local backup of your Address Book and iCal.
That's a big one. Mobile Me doesn't have previous version recovery, change tracking, or notification when something goes wrong. It recently deleted a contact, and I didn't realize it for two weeks. Since Mobile Me contacts are also used for the iPhone, that can be a problem when you need to call your Aunt Margaret to thank her for the hideous sweater that doesn't fit.
I did have a copy from an offline backup, but it took awhile (and too much effort) to extract just the one contact.