Since no one answered here, I posted this question to mathoverflow, where Igor Rivin posted this answer, which I "accepted":
> See this paper of R. E. Miles (he has plenty of related results for points on the sphere, etc, etc, mathscinet will tell you more). The results you want are in section 9 (p. 112, and thereabouts). (the paper is: On the homogeneous planar Poisson point process, R. E. Miles, Mathematical Biosciences 6 (1970).