You CAN share catalogs but you cannot use them simultaneously.
I store my catalog on Dropbox and use it interchangeably between my iMac and MBP...but not at the same time! and you also must be sure that the dropbox files are completely updated before shutting down one computer and switching to the other or you will have conflicted catalog files (a minor mess).
If you are really looking for real time sharing, I don't know of any catalog app that will do that.
Hope this helps...
Thanks SausalitoDog! This works for me. I'm actually signing up for SugarSync 100GB plan for $25 for 1st year (DropBox is $10/mo for 100GB plan) in order to ease image importing while traveling, at least until Creative Cloud allows us to do it.