The standards for SD cards are a little fuzzy, but mainly on the high speed high capacity ones, I think the SPI access ability is optional but very common.
SPI mode on SD cards is not optional. It's optional on MicroSD devices. The SD Association sets the standards for SD Cards, Sandisk is one of the founding members.
Even though SPI is optional on MicroSD devices I haven't found one that does not have it. I have heard of them, but I couldn't name manufacturers or specific models.
This doesn't relate all that much, but I saw this tear down of SD cards and fakes. It was really interesting and has some insights into the industry: On MicroSD Problems bunnie's blog
Those are nice boards on your blog Jason. That clear PCB material is pretty cool.