I think fewer people are aware of it these days when you can simply hit up google:

http://www.google.com/search?q=.003+light+seconds+in+miles

Though it seems they haven't gotten millilightseconds to work.

As always, WolframAlpha is your friend:

http://www.wolframalpha.com/input/?i=3+millilight+seconds

558.8 miles or 899.4 kilometers

They even have "comparisons", e.g. for 30 millilight seconds:

~~ 1.4 x Amazon River length (~~ 6400 km )

I wouldn't have even thought to look for such a program. And this is how I justify reading stories like this :)

Take a look at the data file (/usr/share/misc/units.dat) some time. It's an interesting little declarative non-Turing-complete DSL, in addition to being a handy reference manual.