A convertible SLS is an inevitable successor to the coupe. Very much like the original 1950's 300SL gullwing (followed by a roadster version in 57 I think), the new gullwing will be followed by a roadster around 2012. I read an article where supposedly MB is also releasing an all electric version as well as a Black Series to close up the production.
I would think the doors would open like any other car in the roadster. The chasis would not need redesigned but rather reinforced....I'm sure AMG designers will come up with a solution