In Mac OS the JRE (Java Runtime Engine) is called MRJ (Mac OS Runtime for Java). Don't know if it is installed as standard - you might have to download it from Apple:

http://asu.info.apple.com/swupdates.nsf/artnum/n11927

Should be the latest version

(list 6284, Mk1 S/N 00299 4GB blue [for sale]. Mk2 S/N 080000094 6GB blue)
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(list 6284, Mk1 S/N 00299 4GB blue [sold]. Mk2 S/N 080000094 20GB blue)