The plot thickens. It depends which type of bitstream I send. I have some that I built with icepack -s, so that they leave the flash in woken-up mode, so that XIP from flash memory works.
If I send one of those bitstreams, I get the 0xFFs. If I send one that was built without the -s flag, I get the valid flash id.
<myStorm 0.601> 0x1F,0x84,0x01