With the 2.6.21 I don't need any tweak, at least for me.

First configure the kernel

# cd /usr/src/linux
# make menuconfig


Under Device Drivers I have MMC Support selected, MMC block device driver and Secure Digital Host Controller Inferface Support as well.
Then TI Flash Media as module.

If you are first selecting this option you'll have to reboot with the new kernel.
If not you can compile and load the module tifm_sd. After modprobing you can run update-modules to have that module loaded always.

# make && make modules_install
# cp arch/'YOUR ARCH'/boot/bzImage /boot/'YOUR KERNEL'
# reboot (not needed depending on what you had on your previous kernel)

# modprobe tifm_sd
# update-modules

The device will be named /dev/mmcblk0, /dev/mmcblk0p1 for firt partition and so on.

Now mount, you'll need vfat fs support.