Sync is one of personal computing’s holy grails. Here’s an interesting article on why it’s difficult.
I think that this problem is well on its way to being solved by Ray Ozzie at Microsoft, with MS’s Live Mesh product. Live Mesh may be the next killer OS feature: with Live Mesh, you can painlessly have everything about your PC (browsing history and personal data such as word docs and spreadsheets) automagically synced across all your windows platforms – eg laptop, home computer, work computer, or a computer your visiting, just as long as it’s running Windows. (No cheesy hardware eg Pogoplug required!) Sorry, but it won’t work on a Mac or on Linux. I think this functionality will have network effects and is a difficult enough problem that it might keep people like me on the Windows platform just a little bit longer.