There was a request in the comments for a Spring 3 example on how to do this.
<bean id="systemPrereqs"
class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
<property name="targetObject" value="#{@systemProperties}" />
<property name="targetMethod" value="putAll" />
<property name="arguments">
<!-- The new Properties -->
<util:properties>
<prop key="java.security.auth.login.config">/super/secret/jaas.conf</prop>
</util:properties>
</property>
</bean>