http://wordpress.org/extend/plugins/about/guidelines/
You are probably thinking about #5 and #10 on that page. There are other possibly relevant ones as well, like #7 about phoning home.
Basically, you cannot broadcast your plugin on the front side of the website without the ability to turn it off (user consent). Nor can your plugin be “trialware”. Its OK to advertise, but not OK to be annoying or forceful about it.