Nightly builds higher than version 4.0p (and dated after 1/15/2014) contain POMs that outline each module's dependencies. These files can be used to install both SmartClient and SmartGWT artifacts to your own Maven repositories, where they can be referenced like any other Maven artifact.
<dependencies>
<dependency>
<groupId>com.isomorphic.smartgwt.lgpl</groupId>
<artifactId>smartgwt-lgpl</artifactId>
<version>4.1-d20140115</version>
</dependency>
</dependencies>
The plugin is deployed to Maven Central, so minimal configuration is required. Refer to the documentation at http://github.smartclient.com/isc-maven-plugin/ for more detaied documentation, but the simplest possible invocation might look something like
mvn com.isomorphic:isc-maven-plugin:install -Dproduct=SMARTGWT -Dlicense=LGPL -DbuildNumber=4.0p
which will cause Maven to install the plugin (if necessary) and cause the plugin to download and install the SmartGWT artifacts for the LGPL version onto your local Maven repository.
As always, please feel free to add your own examples here on the wiki or post questions about usage, etc. to the forums.