I had the same error. Don't know why but it works for me using version 10-SNAPSHOT from the snapshot repository
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-shapefile</artifactId>
<version>10-SNAPSHOT</version>
</dependency>
<dependency>
<groupId>org.geotools</groupId>
<artifactId>gt-swing</artifactId>
<version>10-SNAPSHOT</version>
</dependency>
<repository>
<snapshots>
<enabled>true</enabled>
</snapshots>
<id>opengeo</id>
<name>OpenGeo Maven Repository</name>
<url>http://repo.opengeo.org</url>
</repository>