Bukkit

What steps will reproduce the problem?
1. installing plugin
2. start server
3. look in server.log

What is the expected output? What do you see instead?
Expected to see it load. got instead:

2012-02-14 14:42:50 [SEVERE] Could not load 'plugins/RandomNumber.jar' in folder 'plugins':
java.lang.UnsupportedClassVersionError: me/stone_tigris/RandomNumberPlugin/RandomNumberPlugin : Unsupported major.minor version 51.0
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:41)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:264)
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:131)
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:286)
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:201)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:164)
at org.bukkit.craftbukkit.CraftServer.<init>(CraftServer.java:140)
at net.minecraft.server.ServerConfigurationManager.<init>(ServerConfigurationManager.java:52)
at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:148)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:408)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:465)

What version of the product are you using?
version 1.3 with bukkit 1.1-R4

Please provide any additional information below.

Will look into the problem now, been a bit busy with making custom plugins for a server. Expect an update soon that will solve it, have you tried version 1.2.5?

EDIT: Just realised 1.2.5 will not work, but I need to sleep so will fix tommorow hopefully. (17/02/12)