I just read Mikael Åkerfeldt's interview in the latest finnish metal magazine Inferno. The translation goes something like this:
"When I do death metal vocals I want to sound like a fucking monster. I don't want to sound like... a girl haha! I just did a vocal session with another band whose name I cannot mention yet. I hadn't growled for six months and when I came home it felt like it. But what we got on tape - I was like "Wow, it's still there"."
I guess that's it

Mike's coming back!! He also says he has two new songs ready for Opeth. One is very dark, technical and the most intensive song he has ever written. The other one is more simple like a Uriah Heep ballad from the middle 70's.