Try this:
apt-get install evolution/unstable
If that doesn't work, then try this:
apt-get -t unstable install evolution
The first option tries to install evolution from unstable, but doesn't necessarily try to install dependencies from there.
The second option will install it from unstable and grab dependencies from unstable.
Most likely either of these options won't cause you any problems but if you want to simulate what will happen before you install you can do this.
apt-get -s install evolution/unstable
or
apt-get -s -t unstable install evolution
Let us know how it goes.
Adam