The Servo layout engine is Mozilla's experimental project written in the Rust programming language and seeking for a high level of parallelism in the rendering and layout of web-pages. While there's not yet any firm plans to replace Gecko with Servo inside Firefox on the desktop, the new engine continues to be developed and was talked about a few weeks back at LinuxCon Europe 2014 in Germany.
Aside from talking about the usual Servo points with the work done by Mozilla and Samsung, some new performance numbers were shared that show how fast Servo is relative to Gecko when it comes to handling web-pages:
Rendering CNN's web-site was much faster with Servo than Gecko, even when just running Servo with one CPU thread.
Handling Reddit is also much faster with Servo than Gecko and blew way past Gecko when using its multi-threading abilities.
For those wishing to embed the Servo Engine within their own applications for web rendering, Servo seeks to have a stable API/ABI, will be a C-based interface, considered "flexible" by its developers, and has already been largely designed.
In 2015 the Mozilla developers might try embedding the Servo Engine within Firefox Android and Firefox OS, but there's not yet any plans for using Servo to replace Gecko within Firefox or Thunderbird.
The Servo road-map the 2015 plans include releasing an alpha-quality browser using Servo as its rendering engine, landing a Servo component authored in Rust into Gecko, and implementing more features.
The LinuxCon Europe 2014 slides about Servo can be found in PDF form.
Also don't forget that tomorrow Mozilla is expected to unveil a new web-browser targeting developers, which will most likely be Gecko based.