The latest version of the Solar framework has been released, according to this new post on Paul Jones' blog today. Version 0.18.0 has several updates, but most importantly, there's an update to the testing methodolgy.
I can say that the procedural PHPT testing style is easier to approach for someone new to testing, and is great for smaller or standalone projects, but it doesn't seem to "scale up" well for larger class structures. The object-oriented testing style used by PHPUnit and SimpleTest requires more inital setup work, but "scales up" much better in larger projects.
So in a spirit of meet-in-the-middle, I coded up a pair of Solar-based unit-test classes to take over from Solar_Test_Assert and .phpt, then converted a couple of existing .phpt tests to the new technique. It worked like a champ.
He also provides some examples with the inclusion of code from the Solar_Test class, a test written for Solar_Uri, the Solar_Test_Suite class, and a command-line test-runner (as well as a few others).