The problem is not in the RadStyleSheetManager - the article that I sent you contains solutions for various issues including the multilne textbox - please check the zipped file there.
The exact part of the solution that you need is:
| /* Fix problem 2: Needed because IE converts a single CSS rule into three rules, which confused the RadFormDecorator initialization mechanism under IE */
| textarea, button
| visibility:visible !important;
All the best,
the Telerik team