I just wonder if there's a bit of a culture-jam here. It'd tempt you to not open up projects as freely, which seems at odds with the ideals of the GH camp.
The public repo thing is a bit odd anyway. Many public projects don't have a license attached to them. So they're not really open source, they're not public domain, they're just publicly viewable but you have no license to use it.
They'd be nuts to do that. They are getting $7/month from me, for a few private repos, and I bet I'm not the only one. If they up the number of private repos, I'll stick with them, rather than porting my inactive ones to bitbucket. But I doubt they'll make them completely free, even though the competition is free.
They have a great business model - free repos for OSS drives users, then they get users to pay a smallish (in absolute terms) cost for private repos. All they need is the million odd developers who use their system for OSS, and they will shake a few bucks out of each somehow. There's no need for them and BB to compete as though they were selling commodity products. Both are competing on getting people used to their interface (by OSS products) and then charging then whatever they think won't cause too much pain.
That said, people with lots of small repos are feeling unnecessary pain from Github's pricing scheme.
The difference is that BitBucket pricing is based on the number of collaborators. Their pricing page makes it seem like you even have to pay to have more than 5 collaborators on a public repo, whereas with github you don't. If I'm reading it right, then Github is still really good for Open Source projects that intend to have more than a handful of collaborators. While BitBucket is a good place to stick your private repos where you have few (or no) contributors.
I could see GH creating an "unlimited" account for $20/month that allows up to 3mb of files and unlimited repos.