Security is provided via basic 64 or 128 bit WEP encryption, although you can also specify Open System or Shared Key authentication. Linksys says the WET54G will get a Wi-Fi Protected Access (WPA) upgrade, but didn't tell me when that would be.
Unfortunately, try as I might, I couldn't get the WET and the Linksys WPC54G card that I was using for testing to connect when I enabled WEP in AdHoc mode. I also tried a Belkin F5D7010 card with the same results.
Because the WET isn't an Access Point, it doesn't have MAC address Association control, nor does it suppress broadcast of its ESSID. So you may run a higher risk of having unwanted wireless "guests" if you are using a WET to add wireless capability to your network because of these limitations. Another risk is that the WET does most of its neat tricks when operating in AdHoc mode, which I feel is a more "promiscuous" mode than Infrastructure.