1.) Yes, it's a new type of primitive. It is editable in the same way as a sculpty, IE, stretch, position, and rotation only.
2.) It will be TPV's responsibility to backport mesh, although the Snowstorm project will no doubt assist in getting the code out.
3.) The ceiling cap is 65536 verts, although more verts will count as more prims towards land totals.
4.) -Zbrush has Collada export.- Edit: My mistake, it does not.
5.) Other than saving your model as a .dae file, very little preparation for SL is needed unless you want a rigged model, just keep the vert count in check. One thing to note is you'll want to make 4 separate LOD versions of your mesh, with each lower LOD using at most half the verts of the previous one. All four .dae files will be uploaded in unison. The upload dialog has the ability to generate these lower LOD models for you, but you have more control of how your mesh will look from far away if you make them manually. This custom LOD, along with custom UV texture mapping, would be the two major improvements to converting existing sculpts to mesh.