Thanks for your answer, Rex.
Indeed, when I try on a 64 bit machine (sbcl 0.9.8 64bit):
# uname -a
Linux debian 2.6.8-11-em64t-p4-smp #1 SMP Mon Oct 3 00:07:51 CEST 2005 x86_64 GNU/Linux
# dd if=/dev/zero of=/tmp/foo bs=1024k count=2048
CL-USER> (probe-file #p"/tmp/foo")
Now for my particular purpose I only need to be able to list the files, not open them. Is there a better way to do that instead of (run-program "ls" ...) and parse the result?