Drupal 5 doesn't have a drush_backend_fork() anymore. I pin-pointed this to this commit:
I looked at the issue, and the removal of that function is first introduced in this patch:
But there is no explicit reasoning and more importantly no explanation on how to replace function calls to that function.
I'd like to use Hostmaster with Drush 5 but drush hosting-dispatch fails with the following currently. There seem to be only 4 places in Hostmaster where this function is called, two of them in an API file (http://drupalcode.org/project/hostmaster.git?a=search&h=HEAD&st=grep&s=d...) so it shouldn't be a huge amount of work. I don't know if there is anything else to do for Drush 5 compatibility, though.
I'd be happy to supply a patch, I just need to know what to do :)