I think you and I just might disagree on how recognizable this is, but its beside my point.
I get some utility out of the thing, and I get to contribute to something that the community would like to see become successful. I don't normally donate at the drop of a hat, but pg has thus far very carefully tended this community without asking much directly from it. Its a no-brainer.
If HN becomes a Kickstarter for non-profits, eh, I become much less interested then.
Oh, and if someone happens to ask me what the shirt is about, I probably would say "Ah, its a novelty shirt I bought from this geeky website you probably wouldn't care much about." Because I tend to think the community benefits from being careful who gets involved in it. {sigh} I typed that, and its probably flame-war inducing, but I do feel that way.
@mayank same applies, inside joke to who? Those who where told it was a HN Tee or the HN community?(assuming its not recognizable to them).