I work frequently with four different machines. Two desktops, a laptop, and a tablet PC.
I want to be able to use programs in the same way across multiple machines and have state be preserved across them. For some things, I use unison. It works reasonably well for the KDE PIM apps and my Firefox bookmarks.
It doesn’t work well for things like NNTP readers (where article numbers vary from location to location) and aren’t easily fixed.
But then there are things like ipodder. It’s an otherwise nice program, but there is no good way to sync its ~/.ipodder between machines (especially if the download area is not to be synced.) Urgh. It gets all confused and messed up.
I wish more programs would think about this.