After updating, Script.aculo.us effects don't work with Opera 9.0 but work with IE7 and FF 2.0.
It's seems that the Prototype library provided with the beta does not manage correctly Opera. If I replace the Prototype.js file by the one used on Script.aculo.us (and not the one downloadable from the site), it works again, even with the script.aculo.us files of the beta.
So, it's clearly a bug from Prototype, but as symfony provides Prototype and since it will be corrected, I think the team should downgrade to 1.4.
I look to post a ticket on the Prototype website but I can't find the bug report or a forum on http://prototype.conio.net/
I see that Script.aculo.us 1.7 beta is out and work with Opera on the demo website. It use Prototype 1.5RC2 and not 1.5RC1 like the symfony beta 1.0.