They might be just a \r or a \n. I just checked and the text visualizer in VS 2010 displays both as newlines as well as \r\n.
This string
string test = "blah\r\nblah\rblah\nblah";
Shows up as
blah
blah
blah
blah
in the text visualizer.
So you could try
string modifiedString = originalString
.Replace(Environment.NewLine, "<br />")
.Replace("\r", "<br />")
.Replace("\n", "<br />");