(Full disclosure: I am the author of Solar <http://solarphp.com> and have never used Cake <http://cakephp.com>).
My own take on this would be that Cake is more closely related to Ruby-on-Rails, and strives to be nearly identical in features and implementation. It appears that Cake (like RoR) is for building applications in-place on individual systems, but I could be wrong there.
Solar, on the other hand, is more closely related to such projects as PEAR <http://pear.php.net> and Horde <http://horde.org> in that it is first a class library and second a framework. Granted, it's still a framework, but the structure is based on a library approach. The Solar_App structure for applications is superficially similar to Ruby-on-Rails in some respects, but it is not a direct derivative.
In addition, Solar is geared toward easy distribution of applications once they are built, and sports built-in localilzation features. As such, you can build applications in-place on individual systems, but if you build them according to Solar conventions, it becomes very easy to distribute your application and its libraries on a PEAR channel. Adding translations for distributed applications is as easy as creating a PHP-array-based locale file.
One point on which Cake and Solar are very similar is that they both use PHP-based templates for their views. While Cake appears to use PHP directly, with helpers, Solar uses the Savant3 http://phpsavant.com/yawiki/area=Savant3 template system with plugins. I think the functional difference is minimal, but I could well be missing a benefit of Cake in this regard.
Solar is built for PHP5, is E_STRICT|E_ALL compliant, and is committed to unit-testing for all components (alhtough unit tests are only partially completed). I don't know the status of Cake regarding these points.
I hope this helps to illustrate some of the strengths of Solar; perhaps Cake developers can contrast and highlight their own.