Do you mean having the thread available to read for guest, but any attachments only available after login?
If so the easiest way to do this would be to create a new theme and override "thread.ftl".
Wrap the attachment portion like this:
Theme documentation can be found here: http://dev.jivesoftware.com/documents/index.jsp
This could also be done as a plugin but is a little more work. You will need to do the following.
Copy thread.ftl to you plugin make the above modifications.
Override the thread action definition in your xwork-plugin.xml to point to your new template.