As mentioned in this thread or this one, you'll find the RC1 jars in springframework milestone repository:
...
<properties>
.....
<spring-security.version>3.0.0.RC1</spring-security.version>
</properties>
...
<repositories>
....
<repository>
<id>Springframework milestone</id>
<url>http://maven.springframework.org/milestone</url>
</repository>
</repositories>
...
<dependencies>
<!-- Spring Security -->
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-core</artifactId>
<version>${spring-security.version}</version>
</dependency>
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-config</artifactId>
<version>${spring-security.version}</version>
</dependency>
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-acl</artifactId>
<version>${spring-security.version}</version>
</dependency>
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-web</artifactId>
<version>${spring-security.version}</version>
</dependency>
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-taglibs</artifactId>
<version>${spring-security.version}</version>
</dependency>
...
</dependencies>
...
Of course, adapt it to suit your needs. Strangely, it doesn't contain RC2 jars. But if you need a more recent version, you'll find SNAPSHOTS in the repository below (I don't know if it has a "nice" alias) that you can browser through s3browse.com:
<repository>
<id>spring-maven-snapshot</id>
<name>Springframework Maven Repository</name>
<url>http://s3.amazonaws.com/maven.springframework.org/snapshot</url>
</repository>
<dependency>
<groupId>org.springframework.security</groupId>
<artifactId>spring-security-core</artifactId>
<version>3.0.0.CI-SNAPSHOT</version>
</dependency>