JLeuze touches on a point that I'd been considering for a while. There are many suggestions for standards for a plugin, or expected features (such as uninstall etc), so if a plugin meets all of these criteria (maybe see the wltc post), the plugin could earn a seal of approval from WP, and effectively say that it is guaranteed to work with a certain release...if that's possible...

I realise that's at a bit of a tangent to your original question Jeff, but thought it was a good place to bring it up.

@Ryan - I remember reading somewhere that plugins stay plugins because they're not deemed necessary for everyone's WP site to run, like the example of a gallery plugin, and instead the core is a base to be extended. The core cannot be designed to serve everyone's different needs, hence why its core. So in my eyes, it's best to leave this sort of stuff as a plugin. If I need it, I'll download it.