Internet Explorer does not obey RFC2616 when handling the Content-type: text/plain part of the header - it (sometimes) thinks the output contains HTML and tries to download/display the cron.php output instead of displaying it in the browser window. See forum post: http://moodle.org/mod/forum/discuss.php?d=56954.

Possible workaround is to set content-type as text/html and add a <pre> </pre> pair to format. This makes cron output readable for IE users and has no effect on FF users.

Diff against HEAD is attached.