There's a known bug related to the JavaScript API's TextSymbol:
[#NIM048090 Javascript TextSymbol Text property not honoring new line and carriage return characters in the text. ]
My suggestion is to go on http://ideas.arcgis.com and put an idea on the site to or vote for the idea if it already exists.
The only work around I can come up with is to use 2 TextSymbols. I was also able to get IE9 to work but I needed to use IE7 standards.