you have to be on an iPhone for the push feature to work. It won't work on the touch.
what he said^^^^^^
I think it only does push for the phone cause i think the yahoo servers ping the phone over edge/3g telling it to do a "fetch". only "fetching" email when you have some instead of "fetching" at timed intervals.
I had the 16gig touch before i got my 3g. I just set the touch to fetch every 15 minutes. However, i think this caused problems on my wifi network cause it would kick my xbox off of xbox live. ( this could just have been a problem with my network, I did have a friend complain of his iphone doing the same thing with his xbox)