Right, you had a question. The short answer is that your use will probably infringe the camera company trademarks. However, we also think you can probably get away with it.
Why does it infringe? Trademark law prohibits uses that are likely to cause confusion with an existing trademark. There's a good chance that consumers who see hats saying, "I Like Leica" will think those hats originated with Leica. So, the camera companies have the right to challenge your use. (Another way to look at it is to imagine that the hat says "I LIKE THE STEELERS." You can see how the NFL would be all over your business once they learned about it.)
But you're using an everyday phrase ... It's true that you're using a phrase that might be used in everyday conversations. But in this case, you're not using it in a conversation; you're using it on a hat and making some money from it, too. The trademark is what gives your hat its value. Otherwise you would be selling hats that say, "I LIKE _____ CAMERAS." (And we're back to the Richard Hell approach.)
So why do we think you'll get away with it? We can't guarantee this (and we've been known to get it wrong), but the Dear Rich Staff thinks that your use (1) will fly below the radar of most camera companies (assuming you don't try to expand your exploitation beyond the group you describe), and (2) challenging this type of use in which consumers are pledging their support for a product might seem counterproductive to a camera company. After all, the company is getting free advertising every time someone wears the hat.
P.S. (1) Changing the font doesn't make much difference because the companies have registered the names in standard character styles. (2) You're unlikely to get a copyright on a short phrase no matter how fancy the font.