Even though in the beginning of the default email client session at the Ubuntu Developer Summit in Budapest, it looked like it's certain that Evolution will stay as default in Ubuntu 11.10, towards the end of the session things changed and it was decided to stay with Evolution for now BUT switch to Thunderbird as the default email client in Ubuntu 11.10 Oneiric Ocelot as long as it integrates with the desktop.
The Thunderbird desktop integration is not just about Unity (Launcher badge or quicklists) but also E-D-S, contacts as well as better integration with the GTK theme. And of course, there's also the CD space issue.
To see when / if Thunderbird becomes default in Ubuntu 11.10, keep an eye on the default email client blueprint - when all the "work" items are marked as done, Thunderbird should be added as default (the blueprint needs to be updated firstly with todays notes, etc.).
You can read the session notes here.