Seems that its selecting western as the charset, I switched your site from Western (8859-1) to unicode, utf-8, and it posted my comment just fine. (delete it, sorry)
I’m not sure how MT works, as i’ve never used it. so i’d recommend looking around your config/admin panel to find a place where you can set the character encoding. i’m guessing its set to western, more then likely, and every mt site i go to seems to be using utf8 as the default.
http://www.sixapart.com/movabletype/kb/comments/unable_to_sign.html - that has something on mt with utf8, but says to use western. not sure if its just not setting it for the comments page maybe? i say check the config just in case, more then likely, its something simple that is often overlooked. id say just force it to utf-8 and be rid of it.