It seems that the SD/MMC cards get popular for FPGA designs today. These cards are real consumer products and are in effect cheaper than bare NAND chips (for low/medium volume). I would like to summarize the facts I've found in c.a.f  and on various web sites.
SD Cards are now considered for configuration of an FPGA. One of the first projects, done by Antti, uses MMC and load the configuration stream with a small CPLD (21 cells) . Another FPGA loader was done by Arnim Laeuger for a SD card and takes about 50 cells in an EPM3064 . Ad Anuff has used an Atmel ATTiny12 for this task . The design (asm program, eagle library, schmatic) is available at .
However, what's the big win when configuring the FPGA from the SD Card? Cost saving? You need an additional part (CPLD w 64 cells) or an ATTiny12. Both are around $2. That's the same price as for a serial Flash. Flexibility? You need a SD slot on your PC to write the configuration file onto the card.
I think SD Cards are a very fine addition for an FPGA with a soft-core processor to be used as file system. Than it's better to connect the card direct with the 4-bit interface to the FPAG. If you use it also for configuration all above approaches use the SD Card in SPI mode. That maens 1/4 of the available bandwith. And AFAIK you cannot swith to the SD bus mode without powering off the card. And for a file system it can make a difference if your transfer rate is 12MB/s or only 3MB/s .