I noticed in the mailing list that you guys only talked about users who are installing Ubuntu for the first time, not upgrading from a previous version (unless I totally missed it, or misunderstood it):
> We would like to add a checkbox to the installer, exact wording TBD, but along the lines of "Send diagnostics information to help improve Ubuntu". This would be checked by default.
If I understood it right, then you guys haven't decided whether to leave it enabled or disabled by default for users who are upgrading, right?
In my opinion, I think you guys should make it an opt-in thing for users who are upgrading from a previous version of Ubuntu. It leaves a sour thought in people's minds when they get drafted to something they're against, even though they have the option to leave (pretty sure you noticed how this move became controversial and a hot topic right now). I suggest doing a pop up right after the upgrade and reboot, or during the upgrade process, that asks them whether they want to participate (with VERY detailed info on what you'll be collecting) or not. And maybe leave it unticked.
Personally, I'll help out by giving you guys the data you're asking, but there are others who are totally against this, especially the idea of opt-in.