As mentioned yesterday Microsoft released a mandatory update for dashboard and kernel of the Xbox 360 today. Both Dash and kernel go from version 2.0.2241.0 to 2.0.2255.0, but it looks like the 'background download' feature is not there yet. From teamxbox.com and xbox.com:
Read More: teamxbox.com and xbox.com.
*UPDATE* Not listed in the 'improvements', but it looks like Microsoft also tightened the security as they blocked the Kiosk Disc (a DVD sent to shops to run in their Kiosk/Demo Xbox 360 that was found to be signed to also boot from recordable media) from booting so we can no longer experiment with the unsigned data (like music, videos, bf, swf, ... files) on that disc. Several people have confirmed today on our forums that you'll get an "unrecognized disc" error when inserting the disc in an updated Xbox 360.
There's also an article about it on gamasutra.com.
The update is mandatory if you want to be able to sign-in on LIVE.