Considering they are switching to an open source rendering engine, I think there is likelihood they would have open-sourced presto already if that was possible. I suspect that like much proprietary software, there are too many licensed dependencies.
It's a pity, because it'd be very cool to get a source dump of the final presto engine. Even if it was never integrated into a new browser, the historical value alone would be significant.
Opera's response to Open-Sourcing always was, that they would have no business that way. Apple doesn't care I believe, Google/Microsoft are in it for search and ads, Firefox isn't a business (and they make all money from search I think). While Opera makes a lot of money from search, they have a lot of OEM deals that they are afraid to lose.
So if there was no legal constraint, Presto could see day of light. If nothing else, then in patches to webkit :)
