Based on my experience, synchronization is a very difficult problem to solve, and never works fully, hence does not work at all, for you cannot trust your data anymore. Unless you’re using a single OS platform that natively supports advanced synchronization mechanisms through the use of online services, such as Mac OS X with iSync for example, getting it to work for end users is close to impossible. My recommendation is to go for a much simpler solution: get good communication devices that will keep you online most of the time, and learn to do something else when you’re offline, such as reading a book or taking care of your family.

The problem however could be with inconsistencies that occur when for instance your secretary is updating your online calendar and you are planning something as well offline. But I guess one has the same problem with a traditional pen and paper agenda ;-) Perhaps a SVN-alike system to solve conflicts would come in handy…