This was on my MacBook, with both the standard HotSpot Java 5.0 and Java 6.0 preview versions. I also compared using the -J-server (just becomes -server for the JVM) option, since HotSpot on the Mac runs in client mode by default. Thus the -server made a big difference.
This was on my home desktop system, a 32-bit Windows Vista system. I only did native Ruby vs. JRuby with and without the -server option. Again JRuby with the -server option crushed native Ruby.
Finally the environment that caused all the problems, my workstation. As you can see, it did quite poor compared to native Ruby. However, JRuby with either the 5.0 or 6.0 HotSpot JVM was much faster. I was actually hoping to see a better performance advantage on the 6.0 VM vs. the 5.0 one... I don't have IBM's 6.0 VM, so I could not include it. The HotSpot VMs were both 64-bit, whereas the IBM J9 on was a 32-bit VM.