I haven't used Opera much, but the switch to Webkit means that there are only three engines remaining- Gecko, Webkit, and whatever Microsoft is doing...
This isn't that great from security and compatibility perspective- less targets in case of security, and we'll soon see "this page is best viewed with Webkit" type of page design... Monopoly is always bad, and this will reduce competition. Although rate of innovation in web browsers today is quite good, so this probably won't have huge impact.
Webkit already is the new IE, at least in the mobile space. As funny as it may sound, Microsoft has major problems with it because so-called web developers often use CSS with only webkit prefixes, and don't bother with adding the prefixed versions for other vendors or the W3C standard variant of the property.
Open-source monopolies don't hurt innovation speed (it usually improves). It may cause more security runs, which forces vendors to speed-up their update processes. It ends "monopoly-owner tactics" like high prices or bundling-based monopoly growth. It can hurt standards development (because nobody cares anymore).
These people-owned monopolies are better for people.
...if they release Presto as GPLv3+. Else yes, the open web will suffer.
A monopoly (from Greek monos μόνος (alone or single) + polein πωλεῖν (to sell)) exists when a specific person or enterprise is the only supplier of a particular commodity. Monopolies are thus characterized by a lack of economic competition to produce the good or service and a lack of viable substitute goods.
There can be no monopoly in an open source standard because the good is not tied to a single vendor.
IMHO every vendor should agree on a single open standard, which would make the lifes of developers and users much easier. That rarely happens though, a thus forks are born.
The same applies to the linux kernel and other such components. It would be so much better if the stuff that impacts compatibility was all open standards, and the different implementations could change everything, but not touch compatility...