UPD: working with xCode 7 & Swift 2 — Nov 17, 2015
This is a hackathon project done in like 12 hours — expect troubles :)
It builds on iOS7&8 in xCode6.2, but sprites doesn't load on iOS7 — http://stackoverflow.com/questions/24069168/swift-and-spritekit-wont-run-on-device-running-ios-7-1
There's a map for the whole first level. You can use Tiled to edit the level map — http://www.mapeditor.org/ You can find other sprites here http://www.spriters-resource.com/nes/supermariobros/ or elsewhere.
Nodes for collisions load (not at all lazily) from "Collisions" objects layer in the .tmx level map. They are stored inside a node, which moves together with the map. Not all collidable objects are added :)
Collectables (mushrooms, coins) are just rendered where they should be, for now (not by objects, simply by static tiles). There're no enemies.
I didn't have time to process any collisions. Mario can move through obstacles, if pressed by the walls :)
Have fun and contact me if you have issues.