Heh, even the old email client in OS2007 could download just the headers.. and it shouldn't matter if it's pop or imap, pop has always had this possibility. It matters quite a lot when using a gprs/edge/3g connection. I wouldn't be able to use my N800 to read emails if it wasn't that it only downloaded headers, because the cost would have been too high.
It would be baffling if there's not an option to set headers only in the N900 email program. I haven't got mine yet, but someone must be able to comment on this?