Yes, you can then get the tablet either into CM7 from the card or into native Nook.
The details of how seems to be sightly different depending on who-knows-what. On mine, with the CM7 microSD card in, if you boot from a TOTALLY Off state, it boots from the card into CM7. If you reboot from any non-power off state, or if the card is out, it boots into native Nook.
Some people have cards that require power cord to be plugged in to boot into CM7. As I understand it, you cannot tell in advance if yours will work that way.