Google Instant search launched on 8th September 2010 & Google promised Opera will be supported shortly
Shortly is kinda vague it turns out that it means 10 months if you work at Google
Thanks to Opera having the ability to edit any pages preferences you could select the network tab & change the browser identification to mask as Firefox & the instant search worked in Opera as well. Opera support for Google Instant: 'Shortly' | Deep Tech - CNET News
To try it out go to Welcome to Google Instant
You can ignore Google Instant requires Chrome v5/6, Firefox v3, Safari v5 for Mac or Internet Explorer v8.
They will probably shortly
add Opera to the list of supported browsers