You can not share the Eclipse installation directory or the workspace, but the projects themselves are easy to keep in sync using a version management system like cvs, svn, git, etc. I suppose you could store your project contents in a Dropbox folder (or similar file system syncing mechanism) and then just force Refresh when you sit down at a machine that was using those projects, but I've never tried it and would be wary that human error could lead to lost work or corruption of files.
The key is that, although workspaces themselves can't be shared, projects don't have to be located physically under the workspace folder on your file system. That's because the workspace is a logical container for projects, not necessarily a physical container. When creating a project you can specify an arbitrary file system location for the project contents. The default just happens to be under the workspace location. SO on each machine you'd have a separate workspace that imported the project(s) from wherever you are syncing them on your file system. That way the workspace is a tiny container that doesn't require much ongoing maintenance on each machine. I do this locally all the time - I have multiple workspaces on my machine, some of which include the same projects as others.
