It can definitely be a problem. `I have wrestled a few of these out, and had to give up on a couple of others. It's not like you can apply a propane torch to the seat-tube....
Instead of WD-40, try one of the dedicated rust-defeating products. I've used Liquid Wrench with success; there are others. When you apply the stuff, tap on the seatpost with a block of wood. This may help it penetrate.
Instead of pulling, take the saddle off and give it a couple of shots straight down with a hammer. Use a block of wood or something. You don't want to drive the seatpost in deeper, just break it loose.
Apply twisting force to thing. You can use the saddle, or if you think the post is junk anyway, just grab it with a big pipe wrench.
I'm sure some of the other guys may have more suggestions....Good luck!