Comment 1 for bug 227781

libapache2-mod-mono:
  Installed: (none)
  Candidate: 1.2.5-2
  Version table:
     1.2.5-2 0
        500 http://archive.ubuntu.com hardy/universe Packages
        100 /var/lib/dpkg/status

libapache2-mod-php5:
  Installed: 5.2.4-2ubuntu5
  Candidate: 5.2.4-2ubuntu5
  Version table:
 *** 5.2.4-2ubuntu5 0
        500 http://archive.ubuntu.com hardy/main Packages
        100 /var/lib/dpkg/status

If mod-php5 is not thread-safe and has to be built against prefork worker, then there should be prefork version of mod-mono too, to make it possible to run both at the same time.