Do all microSD cards support SPI mode?
According to the Wikipedia article, all families of SD cards support SPI and furthermore, most MMC cards do as well.  In the SD 2.00 specification [PDF], I don't see anything that states that it's optional, though nothing says it's mandatory either.  

However, the specifications seem to keep separate the electrical properties and mechanical form factors (SD, miniSD, microSD), so there should be no electrical difference between any SD card mechanical package unless mentioned separately (e.g. SD vs. SDHC).