Thanks guys. So it's linked to tools that already do aspect calcs.
It's not so much a problem with mencoder, as a problem with doing the aspect calculations more than once (or sending pre-calculated stuff to menc, to recalculate).
I'm doing menc. work with bat files, most of my stuff doesn't need special size work, straight from dvd size to vcd/svcd either 4/3 or with borders (widescreen), as is.
Thanks again.. I just saw a post that referrred to the 'autoaspect problem', and hadn't heard of it before...