One thing we can all do is send letters of encouragement to women to
join wikipedia. I don't know if there is a form letter already used
that we can merge ideas like the below into. This is includes and
expands on points I sent out to a couple of political women friends and
womens lists - about 150 women total - as a personal encouragement.
Underwhelming two responses so far: "good idea" and "I'm too busy." So I
know that the letter needs work! Maybe we could have a couple versions
linked from http://meta.wikimedia.org/wiki/Gender_Gap

I like this idea; and I want to point to some possibly relevant research. The paper "Socialization tactics in Wikipedia and their effects" by Robert Kraut et. al.[1] [2] studies various efforts at welcoming newcomers.

In that research, the finding is that the most effective techniques are those that reflect an engagement with the content that the user has added; in other words, if your "welcome" message is a genuine response to what they did (for example, "Thank you for adding information about so-and-so's history with such-and-such; are you aware of these other similar articles that need expansion?") More generic welcome messages were generally ineffective at getting people to stick around.

It may be that a "call to action" message like you suggest is effective; I guess that's not something this group specifically studied. But for anybody taking this on, I'd suggest that you personalize each one a little, based on the contributor's recent edits, or the info they've put on their user page!

-Pete

[1] PDF file: http://research.microsoft.com/en-us/um/redmond/groups/connect/cscw_10/docs/p107.pdf
[2] Abstract on web: http://acawiki.org/Socialization_tactics_in_Wikipedia_and_their_effects