Lots of brands have been putting text or characters in favicons-- Facebook, Twitter, and Y-Combinator, to name a few. I think it makes the brand more recognizable.

A character or two is not 'text'. And you don't need sub-pixel techniques during image creation to pull it off.

Strictly speaking, if it has no purpose you wouldn't do it. Your hacks may be for learning, entertainment, distraction, avoiding work, etc. Art has a different and distinct purpose, more along the lines of emotional fuel. This is why I disagree with PG about Hackers and Artists. Engineering, design, cool hacks, craft, etc may be exhilarating, but it's not about emotions the same way art is. These are great things on their own, they don't need to steal the word art to be great.

In fairness, in the linked post, only the x-height of each character is 3 pixels. The task the designer of 3x3 set for himself was to create a typeface where each character could only be 3 pixels high and wide at most. That said, the work in the linked post is a very clever and precise exercise in optical illusion. It works wonderfully and is ideal for cases where a design calls for legibility at such a tiny size.