The latest leak from the northern lands contains a whole bunch of developer slides, plus an accompanying (huuuuge) icon archive.
The slides cover such topics as the new UI improvements, new graphics handling, some of the new APIs available for “Super Apps”, and, finally, the new Platform Services that come with the new OS. You can check them out over at Mobile Syrup.
If you’re a keen developer — or just want to have a 1255 BB-themed icons ready for a custom Windows theme — you can grab the icon archive from here. I have no idea how long that link will last, though, so be quick!