We have discovered there is a known bug which has cropped up in Adobe's Flash Player 10 plugin for Mac browsers, where it loses any manual linebreaks from pasted-in text:
This is likely to affect Mac users who have upgraded to the Flash 10 plugin who are copying from another file to paste Variants configurations, paste Stored Codes, or paste preformatted text into an Email Message field in E-junkie Seller Admin. Unfortunately, there is nothing we can do to fix or workaround the problem in the way it affects our Seller Admin. As soon as Adobe fixes this bug, linebreaks in text will suddenly start working fine again for Mac users who upgrade their Flash plugin to use the bugfixed version.
Until then, we can only recommend borrowing a PC to paste-in preformatted text from another file, or just type all your hard returns manually, which (last we read) ~should~ still work fine, as the bug apparently only affects pasted text.
If you are a Mac user and have not yet upgraded to version 10 of Adobe's Flash Player Plugin, you can avoid this problem by holding off on upgrading your Flash Player until after Adobe has fixed this bug.