I've added two new routines to the bit-trick page: 1→4 bit-unpack with reverse and bit reversals. This last one is elegant … except for one bit of C tomfoolery that is required to get GCC to produce the right ARM code. I hope to discuss this in more detail later.
I've also added a new document about dealing with bitfields. It explains what to do with them, gives a few useful functions to get and set bitfields, and demonstrates how to use the C construct for bitfields. It also touches briefly on a nasty detail in the way GCC implements bitfield that can cause them to fail in certain GBA/NDS memory sections. If you're using bitfields to map VRAM or OAM, please read.