The risk is nonzero, but it's hard to tell what level of risk you will find to be reasonable.

Data point: http://unitedtechguys.com/2011/06/10/50000-people-sued-for-d...

Anecdote: at least 1% of the people I went to college with were sent "piracy letters" by the school's ISP while attending school.

Edit: data point from Hn in the last hour: http://imgur.com/vsnmm