Gemstone has just announced an alpha version of their concurrent Ruby engine, MagLev, available for download. MagLev is based on Gemstone's Smalltalk virtual machine and supports 64-bit Linux, Mac OS X and Solaris x86 operating systems. There are no plans for 32-bit version of MagLev.
MagLev does not support Rails yet, but so does not Fabio Kung's JMagLev. However, the advantage of MagLev over Fabio's machine is that Gemstone is determined to create an enterprise-class product, and JMagLev was just a demonstration of the power of Terracotta and does not seem to be developed any further. It seems that the next step for Gemstone will be to implement Rails functionality and allow RoR applications to run in a clustered enviornment, just as Grails ones can run on Terracotta.