In investigating this I also came across the problem
that a 
numeric field set to 0 when that is the very first
edit to that 
numeric field will not be displayed when 
called as:

    <td 
tal:content="structure 
python:context.workingorder.field(size=5)">
      
startdate</td>

I set this to 0, it never makes it through the 
system.
I get an "ok_message=" in the URL, and no auditors 
fire.
If I set it to some none-zero value say 9, commit it (I 
see
that workingorder has been edited in the 
returned
message, and I see output from the auditors) and 
then
change it to 0, it works fine.

-- rouilj