Sadly, as of 10.4.8, it seems scp -Ep still fails to preserve the modification date, at least for any file containing a resource fork. I wish Apple would fix this already! I have come up with a workaround, though.

While ditto was meant for copying files around locally, you can pipe it through ssh to get files off a remote server. (I wrote a short script I call "rditto" which generates the above.) It does an excellent job with resource forks, mod dates, etc., and it even works in 10.3 (though you will need to add --rsrc to the ditto commands in that case).

Apple's provided rsync does keep the mtime however.
And is better suited for what you seem to be doing.

Did you double-check your syntax ? Doesn't seem to work as stated (verbatim)