We study the effect of outdated channel estimates on decode-and-forward relay selection, when operating over Nakagami-m fading channels. Closed-form expressions for the exact outage probability are derived, as function of the correlation between the actual and the estimated channel values. The diversity and coding gain are also studied, revealing a high dependence of diversity order on the aforementioned correlation coefficient. It is shown that when the channel estimates are not perfectly updated, the performance of relay selection is equivalent, in terms of diversity order, to the scheme where only a single relay is available.