Even after upgrading SSH Slaves plugin to 0.5.1 it still reports in plugins as
0.5.1 being available for upgrading and 0.4 as installed.
This seems to be the source of trouble:

• META-INF/maven/org.jvnet.hudson.plugins/ssh-slaves/pom.xml (both in
  ~/.hudson/plugins/ssh-slaves and ssh-slaves.hpi file itself):

<version>0.4</version>

whereas in pom.properties:

#Generated by Maven
#Mon Feb 02 15:47:44 GMT 2009
version=0.4

groupId=org.jvnet.hudson.plugins
artifactId=ssh-slaves

The contents inside seems to be timestamped with 2.02.2009 too, but the file:
https://hudson.dev.java.net/servlets/ProjectDocumentList?folderID=9380&expandFolder=9380&folderID=5818
having 0.5.1 seems to be made on 7.05.2009

This is how it works from automatic upgrade from Hudson.
When downloading directly from
https://hudson.dev.java.net/servlets/ProjectDocumentList?folderID=9380&expandFolder=9380&folderID=5818
I get the correct version.