Have used CustomPress to create a Custom Taxonomy called "Person" and made it hierarchical. It is set to support the post types 'posts' and 'attachments'.
The input box looks just fine in the posts window, but is just a plain text box in the Attachments/Media window.
This is not totally unexpected as it happened the same way when I created the taxonomy manually.
But I know it doesn't HAVE to be that way. I was originally playing with a plugin called PS Taxonomy Expander that does put it into checkboxes or radio buttons if required (second screenshot attached). The plug in, however, errors out (third screenshot attached) when you assign a custom taxonomy to BOTH posts and attachments. It's OK with one or the other, but not both.
So my questions are:
1) the initial behaviour is as expected? No checkboxes for hierarchical taxonomy on attachment pages or is there another setting I could be missing?
2) any chance your developers would add to your plug in what the PS Taxonomy Expander does? (apart from the error part, that is)