When the Nokia Lumia 900 update fix was released, many didn't realize it because by default you have to plug your phone via USB to the Zune software on your PC, which then in turn notifies you of an impending update.
You can get notified directly on your phone about available updates, just turn on the update push notification from settings on your device.
Though this consumes data usage from your phone, and maybe that's why it is turned off by default as it will periodically check for updates. It is best suited for users who rarely connect their devices to PCs, turning on this option is effective, it lets you know right on your phone that you need to update.
Pretty handy option.