I've been doing some testing recently for different subject lines and have noticed that my email client isn't correctly rendering certain symbols/characters.
My subject line is something similer to: "Some great products - all from £99 each".
In Outlook 2007 and Entourage 2008 for Mac (and probably some others) the dash (between "products" and "all") and the pound sign are shown as little black diamonds with a white question mark inside them.
Within the email I obviously use the correct html code for such characters (e.g. pound sign - "£") but I wasn't aware that characters in the subject line were effected.
Any help would be great!
I'd try adding a line within your <head> tags that contains the following:
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
This may fix your problem, however being the subject line doesn't offer any guarantees.