"milli lightsecond" should do it.

"milli lightsecond" doesn't work on OSX unfortunately (under 10.6 at least). One value which does work is "3 c-millisecond".

(I learned this by reading the /usr/share/misc/units.lib file and finding the lightyear definition: "c-yr")

Ah, whoops, I assumed it was GNU Units. Thanks and c-millisecond is cute.

It does, amusingly, know about nanoacres, a unit of measurement sometimes employed when describing chip die surface area.

GNU Units (available in Homebrew) is much more complete than the OS X version.

It still won't let me convert 'bytes / square mm' to my prefered data density format, 'libraries of congress / football field'. So it needs work.

Much better: "2562 units, 85 prefixes, 66 nonlinear units"

Since it took me a moment to figure it out, heres how to install it:

brew install gnu-units

then run as: gunits

So it lives happily besides the default units command.