Execute a Command in PS1 w/o Disturbing Previous Return Value?

[Aaron Davies <agdavi01@xxxxxxxxxxxxxx>]

I'm trying to write a prompt that will do two things: include a file 
frequently re-downloaded from the web, and indicate the return status 
of the previous command. The problem is, if I cat out the file, it 
returns 0, overwriting the return status read by the %(0?%) directive, 
and guaranteeing that I will always get a prompt indicating success. 
What can I do about this?

Here's the whole line. The breakdown is: current temperature, pwd, (if 
last command successful, a green smiley, else a red frowny), prompt 
char.

export PS1='`cat /tmp/temperature`'" %m%~ %(0?."`echo 
'%{\e[1;32m%}:)%{\e[0m%}'`"."`echo \
'%{\e[1;31m%}:(%{\e[0m%}'`") %# "

(/tmp/temperature is refreshed every five minutes by a cron job, in 
case you were wondering.)
--
    __                      __
   /  )                    /  )
  /--/ __. .__  ______    /  / __. , __o  _  _
 /  (_(_/|_/ (_(_) / (_  (__/_(_/|_\/ <__</_/_)_