Nautilus makes some nice thumbnails for media when viewed as icons. I got to like the feature but then I noticed it doesn't happen on Samba shared folders. To get the thumbnails to show, I had to go to Edit -> Preferences. This brings up the File Management Preferences dialog. On the Preview tab there are options that let you set previews such as thumbnails and audio previews for local files or all files. I'd like a few more options in there, like distinguishing between my LAN and the Internet, but so far we can choose between Never, Local and Always. Local will only make thumbnails for files on your computer. Always will make thumbnails for files on your computer or folders on Windows (SMB) shares. Never won't make thumbnails.
After changing my preferences, I didn't see a change in Nautilus for the shared folder I was looking at. As it turns out, I had a permissions problem and I could see that the files existed but couldn't read any of the files. After fixing my permissions problem I reloaded the view and still didn't get thumbnails. I found a note on a mailing list that hinted at deleting ~/.thumbnails. Looking in my home folder under .thumbnails (use ctrl-h to see hidden files in Nautilus) I see there are two folders: fail and normal. Deleting just the fail folder made my thumbnails appear when I refreshed. From the terminal it's
Even after all of that, the largest file that Nautilus will make a thumbnail for is 1GB, so you might still find home movies that don't get thumbnails when you look at them in Nautilus on your file server. I'm sure that'll get patched soon enough.