I have all of my photos on my Desktop PC upstairs, and have just installed LR4 beta on this laptop. When you add files to the catalogue you can chose to leave them in place and not make copies. I have added a few test folders over the network by mapping my images folder as a network drive.
I think if you make changes to an image outside of the local copy of LR you get a little flag on the image saying it has been edited, you can then update the file info from the file or ignore it and use the local info. It is a fair bit slower loading images into develop, but if you only do a little bit of editing on your laptop it shouldn't be too much of an issue.
Just tested this out and it looks like you have to save any changes to file, from Metadata ->save to file for another copy of LR to pick them up.
I don't have write access to my photo folder from here so cant say if it will work, but it should do.