Reasons for Xbox Live Downtime Explained
A website called WhoDigs got in touch with MS, who responded as follows:
WhoDigs: What changes will be made to Xbox Live with the downtime on Tuesday?
MS: This is a service maintenance upgrade on the back end for Xbox.com, Xbox Live and the Xbox.com forums. There will be no dashboard update.
WhoDigs: Any word on what new Marketplace downloads we can look forward to for E3?
MS: We haven't announced anything specific, but expect Marketplace to be flooded with E3 content during and after the show.
So it would seem features such as background downloading will not be coming this Tuesday.