We got confirmation from Boris Schneider-Johne (Senior Product Manager Xbox Platform and Accessories, Microsoft Germany) that a huge Dashboard Update will arrive at the end of may! It will feature a download manager for 6 parallel downloads in the background. He also mentions that there will be a lot more features in there but it will be a surprise and he won't talk about it! The downloads will also running while a game is played.

The user marks a file to download and it will be put in an active queue
downloadlist. There are 6 slots reserved in that queue. They will be
downladed sequentially not in parallel! If the Xbox360 sees there is
bandwith available it will boost the download speed up, if there. During
Online Sessions the downloads will be paused to garantee no lags. It's also
possible to turn off the console, after powering it back on the downloads
will resume. When a download finishes the user will get a message that it
has been downloaded.

The bandwidth managmend looks like to be for lag free chats in the dashboard or some other features which will be implemented additionally, i think so cause during online sessions it will be paused anyway as he mentions in his response to my questions.

Mise à jour du Dashboard fin Mai. Possibilitée de télecharger en arriere plan. 6 telechargement pourront etre programmés les uns derriere les autres pour enchainer pendant qu'on joue. Si on joue sur le live ils se mettent en pause pour eviter le lag. Il reprendrons aussi automatiquement dès l'allumage de la console si ils n'étaient pas terminés lors de l'extinction. A la fin du téléchargement un message nous previendra que c'est fini.

les bruits de couloirs à l'E3 disent qu'il sort au printemps 2007 sur ps3 et fin 2007 sur 360.
y'a bien exclu, mais temporaire :)