What do I need to know about CB seat posts and Al frames when they go together?

Be careful, don't use any grease, don't nick the carbon post, make sure there are no rough edges near the post clamp or seat tube.

Also some posts have a stress relief profile so make sure you're clamping within that region.

Thanks. Now I have an aluminum seatpost and is has a whole lot of grease/lube on it. How do I clean it out from the frame? Should I just get some rag on the end of a long stick and go in there or is there a better way to clean it out?

Rag soaked in some degreaser ought to do it. Be careful not to let the degreaser run all the way down to the BB.

clean it with the bike upside down or on its side that way any excess degreaser will just run out.

Excellent suggestion. I also often take the seatpost off and turn the bike upside down to make sure any water I might have picked up while riding in the rain drains out. Give it some side to side shaking while tilting it too so that any water trapped in other parts of the frame has a chance to make it out. Sometimes water will run down through the top of the seatpost when riding or in my case, it will hit the flat area of the stress-relief section of my carbon post and run down into the seat-tube.

nice, i wouldn't though of that. Will do!