[ATsch]

It's pretty common. If you have the time and space on a board, it's a good idea to fill the silkscreen with useful or important information.

[PascLeRasc]

I love when the info makes it to the shipped version. My bass amp has a miniature schematic drawing etched into itself, it's been incredibly useful. Here's my favorite PCB etching though: http://i.imgur.com/28cYobo.jpg