Berkshire doesn't actually owe back taxes per se. They're referring to deferred tax assets, in my line of work known as DTAs. Some of these are timing differences between GAAP and Federal Tax that may or may not come due based on certain conditions. Furthermore, treatment of certain items may result in creation of permanent DTAs. What the article, and others like this one:
NetRight Daily » Warren Buffett
Are saying is that the firm's consolidated return is being audited and some of these DTAs may not withstand scrutiny resulting in a tax adjustment. Merely because Berkshire may have a billion in DTAs does not mean they owe a billion.
It's one thing to not declare income like Wesley Snipes and another to have tax treatment of items reduce one's tax bill and when audited may not be allowed. Granted, this is somewhat of a complex subject that I really can't expect people to grasp, but it's nowhere as simple as the article you posted makes it out to be.
As for Tim's problem, the author himself admits he forgot about it too. Expats actually have this problem on such a regular basis that every big four firm has a expatriot tax department. There's a girl in my office right now who does that as her entire job.
For those who owe, we should simply garnish their wages. After all, we do employ them.