Well now that's an interesting question.
If this update exists, no official
confirmation yet, it is reportedly an O2 update not Dell. The system updates setting on a stock Streak directs the device to mobileupdate.dell.com a Dell site.
So Dell are not releasing an upgrade which leaves O2 doing something.
Likely a text with a link to follow, when connected via wifi
O2 will supply a link via a website announcemnt
O2 in their agreement with Dell may use the mobileupdate.dell.com site (should be automated from there)
nothing will happen and this is just another red herring.
Cynisism being my middle name I know which one I think it will be...