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It appears to be based on a KHTML engine as seen in KDE products such as Konqueror. According to the site, Safari:
If it takes off it'll be easier to design standard compliant sites without having to hack code for Mac incompatilbilities.
<added>Just spotted this [webmasterworld.com] discussion in Webmaster General on the very same topic, which didn't show up on the site search I did before posting :(</added>