It's currently named icedove-3.0 so you can (for now) install this version in parallel to Icedove 2. This version copies over your Icedove 2 profile so it doesn't get messed up.
As long as the package is sitting in new you can grab it from the pkg-mozext repository:
deb http://pkg-mozext.alioth.debian.org/packages sid/i386/ deb http://pkg-mozext.alioth.debian.org/packages sid/all/
We now need help with going through the Debian bugreports against Icedove.
If you want to help out: grab a bug, check if it's fixed in Icedove 3 and if it is, simply mark it as fixed in 3.0~rc2-2 in the bts:
bts fixed <bugnumber> 3.0~rc2-2
Once we're confident the package doesn't break everything we'll rename the binary package to icedove so it replaces the default Icedove on upgrades.
If you want to help out with patches: Git is here
git clone git://git.debian.org/pkg-mozilla/icedove.git