Just a quick note on the warning. You don't really need the first sentence - the second sentence says the same thing. In warnings, less is more. The longer it is, the more people tend to ignore it, strangely.
It might also be cool to include a list of plugins that are installed but do not support the update check, maybe with a link back to the plugin page as a "manual check" or to contact the developer. That way, the end user knows "Yes, the update checker plug-in sees that these unsupported plugins exist and the update checker is functioning properly, the reason there is no update listed for these is that they are not supported." I don't know if that's possible or not, but it would be more user-friendly.
Great stuff, there. Nice.