I'm not familiar with "adept", but I've seen how RubyGems works. It seems like a very good model to follow for command-line based installers/updaters. I'm sure it using some magic meta programming that can't be ported to php. The meta programming is doable in php. In the end php is a template programming language of sorts. It just doesn't really help you with the task by hiding its strongest features very well indeed. With a bit of hackery you can have lisp-like macros in php, which is quite a powerful tool or a way to shoot your leg from the head down.
Has anyone thought about using the PEAR installer? I heard alot of hype about it in the PHP community at one point, then it died down. I think http://pearified.com gives a list of a few channels already setup. Having a drupal pear channel is a possibility and it should be doable, at least in theory, with the dependecies api. That could be of help to the more adept end-user. This would essentially be a delivery method. Unfortunately we have quite a few miles to go until we are concerned with the delivery.
We could create a drupal channel and have core, it's modules, and all the modules in contribs available through it. Then to install drupal core after they've "discovered" our channel, someone could do "pear install drupal/drupal_core". Yep, it can be done.
This could possibly lower the bar of entry to installing new modules. Following with Bèr's philosophy, a decent web-based module retriever/installer/updater could grow up around it allowing an administrator to update their site without ever leaving it. That is a good goal. But the required infrastructure is not there yet.
Those are my (likely over-ambitious) thoughts on this. Overy ambitious? You don't even reach the half of the Amsterdam pipe dreams we share with Adrian for a while. It's just putting our time where our dreams go is a bit hard.