The title says it all. Pistons can push and pull obsidian. I was under the impression that this would never be the case, so I decided to post a bug report and hope for the best. Just open a world in 15w17a and test it out.
Edit0: Works with and without slime blocks.
This may or may not be related to the issue I recently reported, where slime blocks can also move obsidian now: MC-88429