I guess http://www.addthis.com/ kind of does this and has a "fallback" option to integrate many services.
Mobile is definitely a problem but the mobile version of the website makes that solveable for the most popular devices.
I guess it's a matter of taste but i find the addthis button bar rather horrible to look at. Plus it does not have nearly the feature set that Nic is looking for. I agree there's a product there.
There is a product here - package the buttons together so they only load once, hide them in some way by default, and provide a bunch of analytics. Lots of blogs and websites would pay for that - the current solutions for social button analytics are really bad, or non-existant (you need to know things like click-through rates, which share options are most popular, autohiding some for users who never use them, detect if the user is logged in for each service, how many followers/likes/retweets etc. each user gets from a share so you can identify 'power sharers' etc.) - I don't think that exists.
You can't just keep adding more and more buttons for each social service, and there are some people who will only use, for eg. delicious or pinboard and there are no buttons for them.