*Re* was certainly used with the same meaning, as stendarr points out in another answer, but it was not used in the same manner.  For example, Cicero did not start his letters with it, although there are examples of him using the word with the meaning "in the matter of."

There are many references in ancient texts showing the use of the word *res* in the ablative with the meaning "in the matter of."  The Romans used *res* much like we use the word "thing" and they used it frequently.  Even our word "republic" is from *res publica* which literally means "the public thing."

In a crazy sci-fi world where the Romans had email, I think using *re* just as we do would be completely consistent and appropriate.

Edit: From Cicero's letters:

1. [*in hac re*][1] translated as "[in this matter][2]"

I found abundant other examples among Cicero's letters of his use of *re* which I think could hold a meaning like what we're discussing, but this was the only one for which I was able to match the Latin text to an existing translation.  

These examples are all in the body of his letters as he directs the reader's attention to some matter they've previously discussed.  *Re* does not ever occur as a sort of subject line like we use it.  There's also the example I mentioned in the comments.

  [1]: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0017%3Abook%3D1%3Aletter%3D1%3Asection%3D2
  [2]: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.02.0022%3Atext%3DQ%20FR%3Abook%3D1%3Aletter%3D1%3Asection%3D1