as per my experiance such things could be done this way...
1) it get you rid of lots of escape sequencing stuff..
2) gives you better flexibility on what you doing
3) more redable and managable by other programmers
4) you can check for nulls etc..
I personally always do such things in a function on server side and then call it in aspx page like
<br />
onmouseover = <%# GetToolTipFunction(Eval("Iitemname")) %><br />
in my code behind
<br />
protected string GetToolTipFunction(object itemname)<br />
{<br />
String functionStr = "Tooltip(true,"+ itemname.ToString() + ",event)";<br />
return functionStr;<br />
}<br />
it may look like 2 lines of more code, but trust me it will make your life much easier in long run.