Like PythonMaster notes, dynamic unit conversion would be a bad idea, especially if based on the user locale (since it's very often wrong or misleading). You really want every reader to see the same thing, so that any mistakes in the conversion can be reliably spotted and fixed.
Trying to automatically parse units out of normal text also seems doomed to failure, especially given the existence of common units like "feet" that are also common English words. (You really don't want the system to try converting something like "a horse has four feet" into metric.)
All that said, if we had some explicit markup for requesting unit conversion, and if it was implemented so that everyone sees the same output, then this could be doable and useful. In fact, Wikipedia already has a similar system in place, demonstrating that it can be made to work.
Obviously, I'm not suggesting that we steal Wikipedia's syntax for this — wiki template markup is kind of ugly and awkward, and doesn't fit Markdown very well. But we could come up with our own syntax for accomplishing the same thing; perhaps something as simple, in cases where the default conversion settings are adequate, as, say
[7 miles] producing "7 miles (11 km)".
Obviously, even with the possibility of
stealing leveraging parts of Wikipedia's implementation (or something similar), this would not be a trivial project, and I don't really see Stack Exchange investing a lot of developer time on this (since it's not a critical part of their core business). But if someone were to, say, develop a Pagedown plugin for this, it just might be possible to get it enabled on sites that have a use for it.