I update JQUERY UI to 1.8 rC2 on the app i develop because I had a problem with draggable and IE8 when i used 1.7.2. It was reallt slow. It is really much better now. Thanks a lot :)
...but I find a new problem who occurs only on IE7 :
On my app (see link below), the green button you can drag (criteria on this exemple) are dragging under the widgets. It was ok with 1.7.2 version. It only happens on IE7 (work fine on IE8).
I used the z-index css style and to manage what composant is on top and it works because if you drag a button over the other one, it will be on it, whatever which one you choose.

see the exemple : http://www.mynexthotel.com/test2/template3.html


Thanks for your help.