the official dongles are known to fail after excessive use, as it tends to overheat. well known enough to be mentioned in wikipedia, though with no citation. i do however keep hearing testimonies that the non-official dongles, the cheap ones from china, actually lasts better than MS'.
a sort of fix if you got MS' dongle; http://blog.chase.net.au/index.php/2009/06/broken-xbox-360-wireless-receiver-for-windows/
though if you can, it's much better to get the wired over wireless.