My 500 did the same thing earlier today.
I pressed the reset button on the back until the network light in front flashed two times quickly (that takes about 10 seconds). It then told me it needed to update, which took about 10 minutes and then I had to set all of the settings again (because of the reset) and it worked fine all afternoon.
Just a while ago it said it had an update again, so I said ok, but it has been frozen on the "preparing slingbox update" page for about half an hour now. I am now doing it all over again and hoping for the best.
Thanks DC -
Thanks for your advice on when to release the reset button. I had to hold the reset button for well over 30 seconds to factory reset and reconfigure the device.
After 36 hours I too had the screen locked on "preparing slingbox update - DO NOT remove power". After waiting for well over 2 hours I removed power to reset the unit and everything is now working again. I hope it doesn't lock up any more. I am guessing there is a problem with the most recent firmware upgrade that is causing this issue.