If you're on Windows, I highly recommend SyncBack Freeware as it has far more intensely advanced options on sync operations. You could go further and purchase the Pro version.
If your machines are on the same LAN then online storage is probably not the way to go as you're sending data to the www and back again. So what you would want is a LAN-aware program. Dropbox is capable of both online and LAN syncing. Note that Dropbox is nowhere near as customisable as SyncBack.
You could opt for simpler file transfers and setup Offline syncing if you're on Windows Vista/7. Or something like Live Mesh.
With your situation you're looking at a lot of redundancy (3 places with duplicate data), which is probably not a bad thing in terms of backups. The only thing you have to probably watch out for is how your syncing solution deals with conflicts (example senario: you edit a photo on your laptop, someone else modifies desktop's copy). I've had bad experience where photos/music were lost, misplaced and duplicated unnecessarily. That's when I turned to SyncBack and spent a nice long time configuring every possible option to produce a perfect sync/backup system to match every need I had. I suggest what ever you choose, that you try these senarios to see how it handles it.
Personally I'd let the Desktop read from the NAS and the Laptop sync with the NAS when its back on your LAN. It's simpler and less duplicates. If you're worried about backups, have a separate folder or drive to highly compress and archive them (preferably physically away from your current data, either online or in another room).
Also have a look at similar questions for more ideas.