https://bugs.launchpad.net/linaro-image-tools/+bug/707047
This bug has been marked "fix released" for linaro-image-tools, but there has not been an update of linaro-image-tools in Ubuntu natty since February. If you use linaro-media-create from the bzr branch when writing to the SD card, you should see more memory available.
I am using linaro-media-create from the PPA - given that my host is a Maverick system and this was updated to this mornings version which appears to be 0.4.4
Please note that the cmdline I have at the minute has the following line :
mem=456M@0x80000000 mem=512M@0xA0000000
Thus I would expect there to be atleast 968M of RAM rather than just 662M !
cheers Ramana