hmm, interesting.
If I place your script on a partition of the internal flash (/ or /home) , it doesn't work correctly.
If I place the same script on my ext3 formatted or on a nfs mount, it works correctly.
if you drop the parentheses it works correctly everywhere.
I tried with "test" from coreutils instead of the "test" from busybox, with the same results.
somehow jffs2 seems to cause problems, i don't have an explanation