“HTML 5 is already mature for mobile”
HTML 5 is far from mature on mobile. The support is improving but there are still many unsupported aspects across all browsers. There are also wide variations in performance rendering certain features (such as CSS 3 transforms, animations etc) almost unusable on certain browsers (or devices).

“the mobile web is astonishingly consistent”
There is lots of fragmentation, even amongst single platforms. All Android devices run the same browser, however there is fragmentation across the browsers themselves. Some of this fragmentation is simply related to the various versions (1.5, 1.6, 2.0 etc) while other aspects relate to the actual implementation of each device by a given manufacturer and/or OEM. This pattern replicates itself across platforms as well (Symbian, iOS etc.) Lots of testing is still required and design techniques focussing on flexible/adaptive design are the most successful in terms of obtaining a consistent (yet never identical) result across platforms.

“all devices running a version of the same browser”
This is technically true however, mostly immaterial. There is no “mobile webkit”. Each manufacturer that uses WebKit implements and adapts their version differently. Differences (and bugs) can be due to all sorts of factors including the performance of the device itself. Certain aspects of the latest WebKit are just not appropriate for certain classes of device. The browser is therefore adapted accordingly. (e.g. there is a new touch based WebKit browser on mid-range Nokia featurephones and, amongst other things, this browser supports a lower JavaScript version)

“Opera doesn’t support webkit”
This is true, however there is a bigger picture here due to the goals of Opera and what their browsers are designed to do. Opera has two different mobile browsers. Opera Mini is primarily designed for simpler devices (featurephones). Pages are proxied and compressed on the Opera server, then served to the device. This enables a ‘smartphone-like web experience’ on very simple devices. It also reduceds the cost of usage dramatically due to the data compression. From an accessibility point of view, this is huge and close to 100 million people use Opera Mini each month for this very reason. Opera Mini is also very popular in the US as a significant number (at least 60%) of Americans don’t owen a smartphone. Opera’s web standards support is excellent however Opera Mini specifically, has limitations due to the proxy architecture. Opera Mobile is their second browser, designed for smartphones. It’s standards support is also excellent http://www.opera.com/docs/specs/productspecs/ and uses a proprietary layout engine named Presto.