I'm not exactly sure if this is an issue with the docs or with the QuickTip class, but I'll post it here anyway.

In the docs for Ext.QuickTipManager I found the following example:
HTML Code:
// Add a quick tip to an HTML button
<input type="button" value="OK" ext:qtitle="OK Button" ext:qwidth="100"
     data-qtip="This is a quick tip from markup!"><input>
Above the example it states quite clearly, to prefix the attributes for quicktips with the ext-namespace
To register a quick tip in markup, you simply add one or more of the valid QuickTip attributes prefixed with the ext namespace. The HTML element itself is automatically set as the quick tip target.
Now as you can see, the example uses one attribute prefixed with "ext:" and the other prefixed with "data-".

In the sources (QuickTip.js line 27 the namespace appears to be set to "data-" and if I use the "data-" namespace everything works fine.

I guess its for you devs to figure out if its the sources that are off or the docs :-)