This thread speaks to one of my own greatest concerns. I have some information to add that has not yet been mentioned.
FIRST, I'm evaluating a new facility, still in beta, called Syncplicity, available at www.syncplicity.com
This open-source facility keeps the core files you designate at an online "cloud" hub. You can access it from any computer world-wide via your browser. BUT it also has a desktop-side program which allows you to designate just which files you wish to synchronize. So far, I've got my laptop and desktop in Mexico sync'd up. Whenever I changed a file on one computer, the change would ripple up to the online hub and then back down to the second computer. Worked beautifully.
Now I'm at my other residence half-way around the world. Of course, I brought my laptop with me -- probably for the very last time since the new US Customs policy of seizing laptops willy-nilly, for no announced reason.
I'm planning on setting up a desktop here too, and introducing Syncplicity. Once I set it up, I'll be interested to see if all of my "sync'd" folders get automatically downloaded into that new desktop. I expect it to happen flawlessly.
Assuming my expectations are fulfilled, I'll be one happy computer user. I've been searching for such a facility for 11 years, ever since I established two residences.
BUT THERE'S MORE: I read a couple months back that Microsoft has yet another OS in the works, to follow eventually on the heels of Windows 7, which is expected to be out within a year or two. This new OS has built right into it the facility to maintain core files ONLINE, with synchronized dumps back onto whatever computers one uses. Fabulous idea.
In fact, I see a subtle movement among many software developers in this direction. The new iteration of the free facility EVERNOTE is a prominent example. Also there are new efforts underway to set up "virtual desktops" which are accessible both online and off, all synchronized. I've already experimented with one of these that is in very early beta stages of development. It's promising, but requires very fast internet connections and computers.
I'm beginning to make judgments about which software to buy and install based on similar considerations. I'm beginning to look very carefully at purchase licenses of software I'm considering for purchase. Those that allow installation on only one computer will have a difficult time earning my acceptance. On the other hand, I'll be favorably inclined toward those that allow a single user to install the software on more than one computer, as long as only one is in use at any given time.