I've had this happen 3 times in the last week: Wi-fi on the 8900 was suddenly turned off. When I turn it on by clicking the checkbox next to Wi-fi in the connections screen, there first is a 5 second delay, then the checkbox gets unchecked and Wi-fi is off again.
The good old battery pull works and solves the problem, but then it happens again in a few days.
I had the very same issue, try to change settings to manual selection, I believe under "mobile Network Options" Could also be the router, try a different channel.