Hi, just reporting…
Ubuntu (BB-eMMC-flasher-ubuntu-13.04-2013-06-14.img.xz from http://elinux.org/BeagleBoardUbuntu) can see microSD when running from eMMC.
Yeap, my image has been doing that for awhile. 
It's all in u-boot:
https://github.com/RobertCNelson/Bootloader-Builder/blob/master/patches/v2013.04/0001-am335x_evm-uEnv.txt-bootz-n-fixes.patch
Regards,
I know your debian image on rcn-ee.org.
Thank you for share.
在 2013年6月19日星期三UTC+8下午11时47分15秒，RobertCNelson写道：
It’s curious that your Debian image doesn’t see microSD.
Was it plugged in before power/bootup?
Regards,
I can’t remember. Are you saying it has to be inserted before boot for both Ubuntu and Debian?
yeap... microSD hotplug doesn't exactly work at this time.
Regards,