Came across an excellent javascript tooltip library by Walter Zorn that I thought would be a great additional to my SharePoint sites. Unfortunately follow the instructions on his site would work fine in a normal Asp.Net app but not within the SharePoint environment.
However I did eventually get it working when my grey matter kicked into gear and thought I better blog it quick before senility kicked in and I forgot it 
Edit your master page and add a SharePoint:ScriptLink to the location of the wz_tooltip.js file i.e
underneath
<SharePoint:ScriptLink name="init.js" runat="server"/>
add a new line with
<SharePoint:ScriptLink name="Skullcrusher/wz_tooltip.js" runat="server"/>
obviously just change it to match your location.
This unfortunately isn't sufficient to get it working due to SharePoints quircks and I was getting "tt_aElt.0.style is null o not an object" errors. This turned out to be due to the fact that the tooltip wasn't getting initialized properly and there is a simple fix.
All you need to do is add a call to tt_Init in the onload of the body i.e. change
<body class="body" onload="BLOCKED SCRIPT_spBodyOnLoadWrapper();">
to
<body class="body" onload="BLOCKED SCRIPT_spBodyOnLoadWrapper();tt_Init();">
And that's it you can now use it in any SharePoint page.