For all those who recently acquired a WET54G (rev 3.1), or are planning on acquiring one, should read the problem I had with it.
The problem only affects EU versions of the product, as the US version has a more recent firmware available which fixes the problem.

Update: there's still no new firmware. I've updated the links so they are correct again.
And here's a direct link is here.

Anyway, the main symptom is the lock up of the device as soon as a connection with a wireless network has been made, and that network uses WPA-PSK or WPA2-PSK. I've tested this out extensively, and with different units, so I'm pretty sure it's a bug in the firmware.

Since the EU does not yet have a new firmware (at time of writing), there's but one other solution.

The only way that I found to solve the problem (other than not using WPA or WPA2 at all), is to download and flash the device with the US firmware, found here.

Be sure to select version 3 of the revision number (other versions won't work).

After that, scroll down a bit and download the firmware version 2.07.

Notice

When uploading the firmware, use IE or Firefox, I had trouble with the device not taking my firmware when using Konqueror. After switching to IE, the flash worked the first time out.