Opera recently replaced its own Presto rendering engine for WebKit said that it will follow Google in what it is doing.
An Opera spokesperson trilled to The Next Web that the move will have little influence on the Opera browsers but will affect both desktop and mobile versions.
By adopting Chromium, Opera killed the development of its own rendering engine in order to free up resources to develop new features and build new products.
Opera is a big player in mobile and the switch to Blink means developers of mobile sites will have to ensure they work with multiple rendering engines. Since Blink will be a fork of WebKit, supporting it shouldn't be too difficult, but it means that developers have to think about it.
Blink at least is open source, and if Chrome and Opera signed up it is likely that others will follow.