You have a couple of cool off-mainstream options that you'd like to explore in order to learn more.
Having accessibility and SEO in mind, let's start: make your tooltips in plain HTML and use CSS/Javascript to display them as needed in your requirements.
If you wanna some new, cool and fresh features, you can use CSS3 transitions to display/hide on mouse over/out. It plays nice on Safari, and other new browsers (safari mobile, opera mobile, included).
But for even FF 3.x this CSS3 solution doesn't work, unfortunately.
This let us with some javascript/jquery coding.
Actually, your request resembles me to another very cool stuff: mega dropdown menus.
I've read first about them at Jakob Nielsen's Alert Box: http://www.useit.com/alertbox/mega-dropdown-menus.html
You can find a how-to about them at sitepoint.com: http://blogs.sitepoint.com/2009/03/31/make-a-mega-drop-down-menu-with-jquery/
Well, thing is make a lightweight accessible option with raw HTML and CSS3, and then enhance it with javascript/jquery. You can go further as said and make even some tuning for accessibility on Safari/Android mobile devices (well, safari would work with just css3 solution, so it will depends more on your needs than technology infrastructure available).