As the thread title says, I finally took a deep dive last week into interfacing an SD card to an AVR mega. After studying the SD card specification (which is surprisingly only 263 pages!), it became much less daunting.
I got it interfaced via SPI, started getting the initialization procedure working, and then finally reading block data. Once I had that going, I got Elm Chan's FatFs going to read files. I now have 16 KHz audio playing from an unsigned wav file. There's still implementation details missing; it doesn't handle SDSC cards yet (slightly different initialization procedure) and does not handle file writing.
I felt more accomplished after getting it working (it was not without it's problem, though).