A fundamental problem in dynamic frequency reuse is that the cognitive radio is ignorant of the amount of interference it inflicts on the primary license holder. Policies that attempt to limit interference without the active participation of the primary are thus difficult to implement. However, many wireless systems use flow control feedback such as ARQs. By listening to these control signals, a cognitive radio can obtain indirect information about the interference it generates and thus behave in an acceptable manner. This paper introduces an information-theoretic model of this basic observation and develops and analyzes algorithms that can exploit it. In particular, a simple generic strategy is proposed where the cognitive radio monitors the primary's effective packet rate and only transmits when that rate is above a threshold. The strategy is shown to have important universality properties with respect to unknown time-varying interference characteristics as well as favorable delay properties.