I answer myself for those who will have this pb, there are two
solutions when this error occurs in ie 6 or ie 7 :
1 - you say fuck prototype and you usie native javascript methods :
==> elt.setStyle({foo:bar}) <<>> elt.style.foo="bar"
==> elt.getDimensions().height <<>> elt.clientHeight
(warning with this there are differences between FF and IE)
2 - you absolutely want to use prototype, so you put your element in a
$$ method and magic happens!
In My example :
==> $('search').getDimensions().height return an error "Object
doesn't support this property or method"
And with the magic method IE is happy :
==> $$('#search')[0].getDimensions().height;
GREAT!
....
By searching more , i found it's not so magic. $$ add methods to
elements... For those who have IE DEVELOPER TOOLBAR, you can make a
test by inspecting an HTML ELEMENT that was called by $$, very
nice :D :D :D :D