I have 2 Raspberry Pi's set up as cheap media centers in 2 different places in the house. Since there is no way to route ethernet cables to them I am using Tenda USB wireless adapters with them. Since the Pi (original Model B's, not the new Model B+) have limited current available for the USB ports, and that 1 port is used by a USB flash drive used for the OS and the other connects to a powered USB hub, the wireless adapters are plugged in to the USB hubs. The hubs have their own AC power so they don't overload the Pi's. The hubs currently have the wireless adapters, "powered by the hub" USB disk drives and either a USB IR receiver (on 1 of them) or wireless mouse (on the other). I hope that's enough background.
What happens: the wireless network connections on the Pi's show as active on the OpenELEC configuration screen, but I can't ping the systems and when on the Pi's it's apparent networking has somehow stopped since when I go to weather it never retrieves anything. If I simply disable/enable the wireless on the Pi's then they are visible again and the weather works, etc.. They stay like this for a while, then they just disapear again. At 1 point quite a long time ago I tried putting in a ping to the router for every 5 minutes or so, and while it kept the networking "alive", it messes up playback in XBMC.
The Pi's have a "reserved" IP address in the router, so whenever they come online they get the same IP address each time.
The router side is one of those Comcast "one does everything" boxes - internet, phone, TV.