This is a problem for iso-scan/filename=, too. It gets the list of block devs on the system at the start, and goes through that list, then drops you to a shell. On a fast machine, the boot scripts get to /scripts/casper/20iso-scan before USB devices have shown up. Even the internal hard drive maybe hadn't read its partition table yet. (dual quad-core Harpertown server. :)
I worked around it by adding break=mount to the kernel command line. Then I wait for my USB stick to be detected before pressing ^D at the (initramfs) shell prompt to continue the boot. break=premount is no good, because USB keyboard drivers haven't been loaded yet, AFAICT!
I tried this with Intrepid alpha 4, AMD64 and i386.