That very question was asked and answered today on
microsoft.public.mac.office.excel
(a very useful newsgroup)
Here is the answer
Create a blank workbook set the way you want it - formatting,
zoom, etc. and save it *as a template* with the name "Workbook" (no quotes, no extension) in the Microsoft Office X:Office:Startup:Excel
folder. All new workbooks will use this as their template.
I tried it and it works.