This paper gives an overview of OAI-PMH 1.1 and contrasts it with distributed searching. Note that this paper, and Cliff's below, were written before the name of the protocol had stablilized to "OAI-PMH".
This article discusses some implications of the proliferation of the OAI-PMH.
The above two papers (published well after OAI-PMH has been widely adopted) discuss how search engines are not using OAI-PMH to discover resources in the deep web.