By the default SMS app, no there is no way to mark an SMS as unread.
But there is an app (or apps) in the Market that can do this for you, or at least, should work for you. From a simple search this app is called "Mark as Unread" by Christian Skogsberg. Try it out and hopefully it works for you.
If that doesn't work, or you want to try something else, you could try a different SMS app such as Handcent SMS, I read but cannot confirm you can do it in this.