There are now two versions of the Arnold User Guides. For example:

Arnold for Maya 4 User Guide

Arnold for Maya 5 User Guide

If you are not sure which version you are on, have a look at the top and it should say which version, for example: arnold for maya user guide 5

Also, the url for Arnold 5 has an A5 prefix, like so:

https://support.solidangle.com/display/A5AFMUG/Arnold+for+Maya+User+Guide

A tutorial that covers how to use the Flakes shader to create a glitter makeup effect can be found for the following plugins:

MtoA     MAXtoA     C4DtoA     HtoA

Here is a useful list of render farms that support Arnold.

John Paul Giancarlo has recorded an Autodesk webinar showing some tips and tricks in Arnold for Maya.

In this video, Chad Ashley from Greyscalegorilla demonstrates how to use the Curvature Map inside of Arnold for Cinema 4D (C4DtoA).

If you want to know how to fake a rounded corners shading effect, jump to the end of the video.

Localisation links for Chinese and Japanese have been added to the following documentation pages:

Romain Chauliac has a tutorial that shows the workflow from Substance Painter to Arnold 5.

Here is an animation test I did using the same technique as these still images, rendered with Arnold.

A Lamborghini Murciélago model is now available as part of the Car Studio Lighting scenes on the Learning Scenes (MtoA). The car model is kindly provided by Pedro Fernando Gómez.

An Introduction to Light Path Expressions tutorial is available for the following plugins:

MtoA     C4DtoA    HtoA