The machines running off it are a combination of Windows XP (SP1, SP2 and SP3), Server 2003, Vista Professional and Windows 7 Professional. Nothing has been changed on any of these computers and nothing has been changed on the DGN2000.
The wireless is set up to use WPA and I've also set it up to only allow specific MAC addresses to access it. Using the diagnostics on the DGN2000 doesn't show anything and only our computers are attached to it, so that at least rules out the connection being hijacked.
Any pointers on whet else I can check or any other tests I can perform?