Ian,
I often thought it would be easier to set something like this up by creating a one-column table with a row for each column, and the row height matching the appropriate column width in the main table. In Pages, where a table can be rotated, it works pretty well, but in Numbers, you can't rotate tables.
You can, however, use Preview to convert a Numbers '09 table into a pdf graphic that can be rotated.
A while ago, I played around with writing an AppleScript to automate much of the process, but I wasn't really excited about the results (as far as I can tell, AppleScript doesn't interact with cell borders, so you are stuck with default borders.)
Since SG and some others have been reviving an interest in AppleScripting Numbers, I thought i might post a link to the script, which I updated to work, as far as it goes, with Numbers 3.1. It takes the text values, text attributes and background fills from the first row of a table and produces a pdf graphic, on the sheet, which can be rotated 90° and placed over the first row for vertical headers.
If somebody can use it, maybe as a starting point for a more complete treatment, here it is:
https://www.dropbox.com/s/co4zvs74guh5y5n/vertical_headers.scpt
Jeff