I personally wouldn't be in favor of it.
Addition: Arguing that "Ruby isn't English" and then stating conformity to mathematical notation as a benefit seems hypocritical. Programming languages (besides APL, maybe) do not use strict mathematical notation, and we shouldn't want them to.
I do disagree with the author on one point: 'not in' should also be a valid operator, representing '∉'.
But I don't use Ruby; I mostly use Python, which has these operators.
I can't think of any modern programming language that isn't unicode aware even though for the standard library methods, there are very good reasons not to use them (I know, Perl 6 does but even in this case there are ASCII fallbacks).
You should also have very good reasons to put methods on the root class of your class hierarchy.
Making it easy for people to type them, on the other hand...
That's a problem of the code editors people are using. ;-)
I'm often surprised with how dysfunctional editors people are programming.
For example if all editors would show tabs and spaces (or whitespace in general) in a reasonable way, we wouldn't have had that much pointless discussion about the correct indentation whitespace character.
And as he points out, 'in' is already a keyword anyway - so if it makes a more readable way of doing things, then why not?