[tenshi]

var timerId;
element.onmouseover = function() {
    timerId = setTimeout(function() {
        alert(1);
    }, 2000);
};
element.onmouseout = function() {
    clearTimeout(timerId);
};

правильно так:

var timerId;
element.onmouseover = function() {
    if( timerId ) return
    timerId = setTimeout(function() {
        alert(1);
    }, 2000);
};
element.onmouseout = function() {
    timerId = clearTimeout(timerId);
};

или так:

var timerId;
element.onmouseover = function() {
    clearTimeout(timerId);
    timerId = setTimeout(function() {
        alert(1);
    }, 2000);
};
element.onmouseout = function() {
    clearTimeout(timerId);
};