I also use LUKS and have been seeking a method to encrpyt a partition (I heard doing the whole MyDocs parition is not possible).
Considering the amount of personaldata I keep on there, it would be all too for a thief to get to my stuff.

I doubt you would be able to use a USB stick to store the keys, but you could always store them on the SD card.

Right now the only method I've been able to use is storing my personal stuff on the SD card and using the truecrypt port to encrypt/decrypt; this is not really ideal as I like to use the SD card for other things, and it is not that big.

I really think encrpyption for a device like this is vital.