the current left / right arrows were actually born out of experience. 
The right arrow has always been there, but - if you look at many of 
the early posts to this forum - you'll find that they did not know 
about pressing the left button to get up a level. That was the point 
where I added the left arrow to the menu. Since then, questions about 
this have become more the exception than the rule.

I think an up arrow on the left and a down arrow on the right would 
be confusing. At least it would confuse me ;-) .. plus at least right 
arrows are also used on the iPod.

If the menu levels would visually come and go to the fore-/background 
of the menu and thus there would be an up / down analogy people could 
visually connect to, I'd probably embrace your idea in a heartbeat.

I guess Apple would be one of the very few, if not the only company, 
that I'd consider selling to - if they would mind to ask that is. ;-)

The wealth that I'm aiming for in life is not related to money, 
though .. good health, real love and friends are so much more 
valuable - all things that can't be bought for any amount of money.

Cheers, 
Felix