I have been using Lightroom 3 across multiple machines (as allowed by the licence) and the feature I most wanted was to be able to share the same lightroom catalogue across the machines. I have a mechanism for backing up images to multiple drives, where the directory structures stay the same, and therefore each machine can access the same structure, however nothing to keep the collections in sync. Especially the publish services, which cannot be imported into new catalogues. After a bit of trial and error, Dropbox seems to be the solution. By using the selective sync option to remove the xxxxx.lrdata folder, only the catalogue xxx.lrcat file, and backups are synced to dropbox. Simples.

It is probably best to add a blank xxx.lrdata folder to dropbox first, then selectively unsync it, then put your xxx.lrcat file and xxx.lrdata folder in (assuming you are moving it from somewhere else).