I've used QTip with fullCalendar and it's working great!
$('#calendar').fullCalendar({
...
eventRender: function(event, element, view)
{
element.qtip({ content: "My Event: " + event.title });
}
...
});
Just make sure you're defining your qtip in fullCalendar's eventRender event. :)
The only issue I've noticed (w/ JQuery 1.3) is that when the qtip popup fades-in, it starts its fade-in behind fullCalendar's styled grid. After that first ~few frames, it's fine. Also, this could very well be a problem with some other stuff I have going on in my project. I'm too lazy to debug it further so your mileage may vary. ;p