I have CM 7 and have tested booting on two NTs that I have. I also have 32GB, 16GB and 8GB SD cards (each created from the same image and then increasing the 4th partition to full capacity for the card), and each behaves the same as the other depending on which NT I use. One NT always boots directly to CM7 using any of the cards, the other does not. However, if I first tether the one that doesn't normally boot to CM7, and immediately remove the tether when the first screen comes on, the unit will boot to CM7 (as expected) but will the always boot to CM7 again without tethering (even after a reboot to NT mode, a shutdown and then a new power on) until the unit is charged. After a charge I then have to re-tether the NT once, removing the tether quickly at power on, and from then on tethering is not required (again, until the next charge) just like before.
This behavior is fully repeatable for me and has been checked over the course of several weeks. It's magic I'm sure.