Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
From a posting in uk.comp.sys.mac (http://tinyurl.com/mwng3 )

Elliot Roper in uk.comp.sys.mac said:
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.

My emphasis added above

Change "Microsoft Office X" to "Microsoft Office 2004" or whatever the folder is called as needed.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.