As already reported in issue 6597, which I cloned (7287) hoping to add a comment (I couldn't, so issue 7287 is not really useful - sorry about that), automatic installation of JDK 6 update 21 triggered a reboot, without any prompt or warning.
I'm aware of the REBOOT=Suppress parameter present in hudson.tools.JDKInstaller. It's just that its behaviour is documented in MSDN as "If there is no user interface, the system automatically restarts at each ForceReboot."
It seems that the value "ReallySuppress" may do what it is needed here, though I haven't tested it myself. I just wanted to report this, in case it might avoid other surprise-reboots to unaware administrators.