Just saw this on Amazon and think it is a great price for the device.
Drobo + DroboShare (Network Attach) £356
The Drobo is a 4 Drive bay raid system (1 Disk redundancy) that automagically adds new/bigger drives into the system. As long as you only replace one drive at a time. The Drobo Share adds NAS, takes the Drobos usb interface and makes the drive avaliable on the Network.
I still think the Drobo Pro version will be much better investment in the long term as it has 8 bays and can do double disk redundancy but does cost £970 with out drives and is not NAS (Network Attached Storage)