[mercurial]

git annotate?

[ctdonath]

I'm talking code that has out-lived several "yeah we'll use this forever" version control systems. If the comment isn't in the code it will get lost.

[ZoFreX]

FWIW you can actually migrate between most major VCS systems without losing your history.

[Someone]

Yes, but it is neigh impossible to migrate from say Python to C, a copy-paste into an entirely different project, and from there via Java to C# and keep history. That may be a stretch, but even one such step will almost certainly lose history.

[mercurial]

It also depends on what kind of stuff you've done with your VCS. "Creative" SVN layouts may be difficult to merge.

[ctdonath]

That's assuming somebody remembers where it was.