I have the same issue in Logic 9.1.1. It annoys me too ;-)
Toggle zoom works well to view entire project when no region in selected, but it doesn't toggle back to previous zoom level for me.
If a region/(s) is selected then it works as it should. Though I used to have some success with selecting all regions (command+A) and then toggle zoom (Z for me)... but this appears to not return to the previous zoom level either :(
I like to use the Save as Zoom 1/2/3 & Recall as Zoom 1/2/3 key commands as I find these take me to the zoom levels I find more relevant.
Shift+Control+1,2,3 respectively to save zoom levels with 1 being zoomed in v.close, 2 zoomed in to 8 bars and 3 zoomed to view a sizeable amount if not all of project...
Hope that helps,