A customfield (e.g, customfield_10000) that is applicable to issue types (e.g, subtask type) will only appear on the Subtask issue creation screen, but not in the quick subtask creation form. It does appear if the all issue types are removed from its context. Below is the DEBUG log:

2008-06-12 21:51:28,744 http-8123-Processor1 DEBUG [jira.web.util.SubTaskQuickCreationWebComponent] Custom field with id 'customfield_10000' is not in scope for project 'Testing'.

Note: The customfield is added to the "jira.subtask.quickcreateform.fields" property in jira-application.properties

I have looked into the SubTaskQuickCreationWebComponent class and found that issue type id will not be past to the CustomFieldImpl.isInScope() method.

protected boolean isFieldInScope(OrderableField field)
    {
        if (field != null)
        {
            // If we have a custom field, then ensure it is in the correct scope.
            if (fieldManager.isCustomField(field))
            {
                CustomField customField = fieldManager.getCustomField(field.getId());
                if (customField.isInScope(authenticationContext.getUser(), subTask.getProject(), Collections.EMPTY_LIST))
                {
                    return true;
                }
                ...
                ...
    }

This could be the root cause.