When to create a subpackage or just another package?

[jazz_snob]

Here is my scenario:

For my own selfish reasons I'm writing an MVC framework in PHP5. There is a package containing only interfaces called "Framework". Then there is a package that represents a working implementation called "WebKit". WebKit package clearly depends on Framework.

My question is: should WebKit be a subpackage of Framework or a separate package altogether. In the end it probably doesn't matter because either way the same thing will be accomplished. I'm curious if there is a rule to direct these kind of decisions. I've not been able to find any, however, in my research, with the exception that The granule of reuse is the granule of release. Only components that are released through a tracking system can effectively be reused. This granule is the package

I intend on releasing this open source (BSD license) w/ a complete demo (guestbook application, similar to the Zend framework demo application) in the coming weeks.

Thanks in advance for your advice.

[NogDog]

My instinct would be to have WebKit be a sub-package of Framework, based on the idea that it is a special case of Framework (assuming I correctly understand things). But that's just a "seat of the pants" opinion and not based on any accepted "best practice" that I know of.