Leaflet says it supports EPSG 4326 (Plate Carree, aka ll84) projections. I think it does, but take a look at these two jsfiddles:
http://jsfiddle.net/drysg/cBzVc/ - This is the original 3857 web Mercator. As you can see the overlay of both the tiles and the red outline are properly placed near Kraknow. (click the mouse on the map and you get the lat/long as a popup).
Ok, now look at:
http://jsfiddle.net/drysg/XPP54/ - This is in EPSG 4326 (ll84).
The background is a MapCarta ll84 slippy tiles. It seems that leaflet does not know about the 2x1 aspect ratio that OSG wants. That is why the background wraps (http://wiki.osgeo.org/wiki/Tile_Map_Service_Specification#global-geodetic) Well, if they want to support that fine. I am actually happier with with a 1x1 map (I can generate that with SAFE FME). So ignore that for a while.
Switch the background to the CloudMade OSM map (ingore that the origin is at 85N, so that the map is 5 degrees shifted to the south).
Now everything looks correct, to me.
The only thing I need now is to figure out the formula for the SLIPPY tile numbering. The web page at http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames
Shows how to do this for Web Mercator. What would the formula be for LL84, where the map bounds are not (-85, -180; 85, 180) but rather (-90, -180; 90, 180)