LineBreak is not supported by SVG (more on that - http://www.w3.org/TR/SVG11/text.html#Introduction) and RadHtmlChart uses SVG to render in modern browsers. Add the ability to handle long label text in SVG as well. Currently long strings are getting clipped. 

For the time being the labels can be manually split in separate text elements. For example:
		<script>
			function pageLoad() {
				wrapLabels();
			}
			function wrapLabels() {
				//Find all text elements on the page
				var textElements = document.getElementsByTagName("text");
				//Iterate through the text elements
				for (var i = 0; i < textElements.length; i++) {
					var currTextEl = textElements[i];
					//Set the desired string/symbol that represents the line break
					var lineBreakSymbol = "@LineBreak@";
					if (currTextEl.textContent.indexOf(lineBreakSymbol) != -1) {
						var lines = currTextEl.textContent.split(lineBreakSymbol);
						//Adjust the x and y coordinates for the new text element from the old one
						var textAlignCoefficient = 7;
						var x = currTextEl.attributes.x.textContent * 1 + lines[1].length * textAlignCoefficient;
						var y = currTextEl.attributes.y.textContent * 1 + 20;
						//Obtain the same style for the new text element from the old one
						var style = currTextEl.attributes.style.textContent;
						//Add the new SVG text element
						addSVGTextElement(currTextEl, x, y, style, lines[1]);
						//Adjust the text and the position for the old text element
						currTextEl.textContent = lines[0];
						currTextEl.attributes.x.textContent = currTextEl.attributes.x.textContent * 1 + lines[1].length * textAlignCoefficient;
					}
				}
			}
			function addSVGTextElement(placeHolder, x, y, style, text) {
				var newText = document.createElementNS("http://www.w3.org/2000/svg", "text");

				newText.setAttributeNS(null, "x", x);
				newText.setAttributeNS(null, "y", y);
				newText.setAttributeNS(null, "style", style);

				var textNode = document.createTextNode(text);
				newText.appendChild(textNode);
				placeHolder.parentNode.insertBefore(newText, placeHolder);
			}
		</script>
		<telerik:RadHtmlChart ID="RadHtmlChart1" runat="server" Width="600px" Height="400px">
			<ChartTitle Text="Chart title @LineBreak@ more text in title">
			</ChartTitle>
			<PlotArea>
				<Series>
					<telerik:ColumnSeries Name="Series 1">
						<SeriesItems>
							<telerik:CategorySeriesItem Y="30" />
							<telerik:CategorySeriesItem Y="10" />
							<telerik:CategorySeriesItem Y="20" />
						</SeriesItems>
						<LabelsAppearance>
							<ClientTemplate>
Category is #=category#@LineBreak@Value is: #=value#
							</ClientTemplate>
						</LabelsAppearance>
					</telerik:ColumnSeries>
				</Series>
				<XAxis>
					<Items>
						<telerik:AxisItem LabelText="item 1" />
						<telerik:AxisItem LabelText="item 2" />
						<telerik:AxisItem LabelText="item 3" />
					</Items>
				</XAxis>
			</PlotArea>
		</telerik:RadHtmlChart>