BackPress is a basic framework distilled from WordPress’ core. It handles permissions, plugin architecture, data sanitization, XML-RPC, etc, etc.
First impression after download: it’s young and lean. At this point, don’t expect a full-fledged framework like Django or CodeIgnitor. Instead, it’s a large collection of PHP5 classes that handle a lot of common problems, batteries not included.
I’m betting as BackPress rounds out it will become the separately-maintaned “engine” for WordPress. This modular approach can speed development and raise code quality, since it forces abstraction. ExpressionEngine does this with the aforementioned CodeIgniter framework. jQuery is also developed this way, versioning and distributing its Sizzle selector engine separately from the mother framework.
The timing couldn’t be better. Automattic has announced that the WordPress and WordPress MU codebases will be merged into one for the next release. That makes this an opportune time to clear the underbrush from WordPress’ very organic codebase.
See the last paragraph of the 2.9 release announcement.