Am using the contact_form plugin. Message field cannot be filled in in IE but works fine in Firefox. Try for yourself here: http://www.4seasonscreationsphotos.com/zenphoto/index.php?p=contact
I really do not know what to tell you. I have just started up my Windows XP virtual machine, visited your page with IE6 and send a contact.
I have received the copies of the contacts I posted via IE9, so I know that worked. (I clicked on the field both times.) But just now I have tried tabbing and it takes me field to field no problem.
I suggest you try compatibility mode in IE. IE is a strange beast. Who knows what problems lurk therein. The html all looks proper.
You can also check the page via http://validator.w3.org/check?uri=http%3A%2F%2Fwww.4seasonscreationsphotos.com%2Fzenphoto%2Findex.php%3Fp%3Dcontact&charset=%28detect+automatically%29&doctype=Inline&group=0
it generates no errors.
Ok, I renembered I still have IE8 on my laptop, so I have tried it. And as you have observed, I cannot get to the message field. But if I turn on compatibility mode every thing works. So some kind of IE8 native bug. You could probably report it to Microsoft, but I suspect that will go on deaf ears, specially since it seems fixed in IE9 and you have a work-a-round.