The pipe is for draining rainwater from the bottom of the exhaust stack. 3.6 sieverts/hour radiation was measured at several meters from the exhaust stack. TEPCO has designated the area around the pipe as off-limits.
Japanese Story here: http://www.nikkansports.com/general/news/f-gn-tp0-20110804-815700.html
On a personal note.. If you have a look at the picture, them signs are kinda small are they not? Like if anyone ever would go up and try and read them what's gonna happen. And what do they say?