Hari Karam Singh has put together this comprehensive quick reference card for the new Objective-C literal syntax including a few caveats which you may want to think about when using this new syntax.
Mike Ash took a slightly different tack with his Friday Q&A last week and took a tour around the CommonCrypto library included with iOS covering hashing, HMACs and Encryption.
This may be my new answer to the question "Can you recommend a C tutorial?". Uli Kusterer's guide to C programming takes you step by step through writing a non-trivial C program. This seems to have been around a while but I came across a link to it this week as it as just been updated for Xcode 4.4. I have to say I love the site name as well.
Tom Dalling with a good warning on the dangers of performSelector: when returning structs rather than objects.