Well spotted, Lazer. Hadn't seen it yet. It's alongside of the Dell logo, which has been up there for a long time. Interestingly the Opera logo is not on the homepage, but at the bottom of the advanced search. Well, Opera has been friendly with ATW for a long time, perhaps it's just a way to say thanks.
My understanding is it's a rather simple, lowkey marketing agreement. Opera has for a long time integrated ATW in various ways into their nrowser options, like being present in the search bar, offering the ATW sidepanel, the supersearch, the automatic search from the address bar.
Both companies have been committed to standard compliance. Both play in their field the role of David vs Goliath, with MS and Google.
I don't think there's much more to it. Definitely no partnership on ownership level or any such thing. Note that the Opera logo is not on the default start page.
P'raps not but I personally favor more of these types of arrangements from the standpoint of robust compatability. Even a little bit towards a more compliant/cross-browser world is better than what we have now. At least until everyone decides to get on-board with a common set of standards. :)