Okay, enough about what tone to adopt. What do you say to convince them that the Linux community is worth porting for? Here's what I did.
First, I explained as well as I could just who the Linux community is, and why Linux users would be a receptive group for my target company's products. I very carefully didn't exaggerate, and made sure that I explained when my figures were estimates rather than hard numbers. If someone comes up with a way of measuring usage, we may be able to get the hard numbers we need for market demographics, but till then we have to do our best.
Next I explained why I thought the Linux market would be good for the company to enter.
Then I laid out the ways in which the company's current program line could be ported to Linux. I must say that I lean toward using Abacus Research & Development Inc. (ARDI) Executor technology as a "wrapper" for Macintosh binaries as the easiest and quickest way to do that, but WINE and the TWIN library (as I understand it,these two groups are now working together) are all possible tools to help move programs to Linux without full-blown ports. Incidentally, I cleared my letter with ARDI before I mentioned any action that their engineers might be able to take for the company. You would lose credibility if the target company acted on your recommendation, contacted someone like ARDI, and was essentially told, "We don't know what you're talking about."
There's also Loki Software, which does ports of commercial software to Linux. So far they're done only games, but when I talked with the president of Loki a while ago he was quite willing to consider doing similar ports of other types of software.
Finally, I ended with a personal note on why I was trying to convince the company to port to Linux. Here's a copy of my standard letter; don't copy it word for word, but feel free to adapt its organization if you like: