This is Dyson's "eternal intelligence" scenario. A summary can be found at [1], but the details are in "Time without end: Physics and biology in an open universe" [2]. wcoenen very helpfully linked to a text version of this paper [3].
[1] http://en.wikipedia.org/wiki/Dyson%27s_eternal_intelligence
[2] "Time without end: Physics and biology in an open universe", Reviews of Modern Physics, Vol. 51, Issue 3 (July 1979), pp. 447-460