Good point, quid pro quo -- I hadn't thought about the letter, as I was thinking of photos -- but yes, while not literally "in person" I'd say he signed it. The signature also looks like the one on the Columbo drawing that made and he sent. Here's the letter:

Thanks so much for the help. I attend a lot of autograph conventions and I'm taking the Peter Falk as Columbo signed photo around and having all the "Columbo Killers" I can get also sign it...I've got Martin Landau, Robert Conrad, William Shatner and DIck VanDyke so far. Now that I trust the Falk autograph, I feel a lot better about it. Thanks again! (BTW - Mr. Landau was excited to talk about the episode and his great experience working on the show - what a nice man!)

Just picked up a copy of Peter's book (Hardback / USA edition}. Would love to know if people think the signature is genuine... (that's it in my avatar!)