The strange thing is, this only happens if someone sends me the message; if I start the message they can reply and I can reply back. No one else has mentioned this, so I'm assuming it's something local to me.
I'm using Windows 7 Professional 64bit, Chrome 14.0.835.186 and/or Internet Explorer 8; I've not tried this in Firefox yet, but will later today. Computer is Quad Core Intel with 8GB ram, XFX GForce GTX 260 graphics. Not sure if any of that helps.