0.6.6 is a relatively minor release that doesn't add any new exciting functionality, but clears up a few things which have been long overdue.
Firstly, Luminous should now work on PHP5.2.0 and above, with the caveat that early versions in the 5.2 tree will generate warnings under strict mode. This does not affect later 5.2 releases. This should be a plus to anyone forced to still support the whole 5.2 tree.
Secondly, the cache has slightly better error logging facilities. By default, Luminous is set to 'verbose' mode and will generate a PHP warning on a fatal cache problem. This is now configurable (via luminous::set('verbose', false). Furthermore, you can now more easily log cache problems for 'private' viewing (i.e. not broadcasting debugging information to your users) with the new luminous::cache_errors() function. See the cache docs for this one.
In the interests of full disclosure, a warning: Since 0.6.5 I've been working full time and have less time to spend on Luminous. Therefore with less testing time, I can't guarantee that 0.6.6 is quite as stable as 0.6.5, so more importantly now than ever, if you find a bug, please report it via GitHub.