Thom has a nice note on ETD 2008, held at The Robert Gordon University in Aberdeen. (ETD - Electronic Theses and Dissertations.)
He notes our ongoing involvement in NDLTD:
As part of OCLC's support to NDLTD we harvest metadata about ETDs, and make a union catalog (which includes metadata from WorldCat) harvestable. This is starting to be a sizeable file, with nearly 750,000 records in it right now. [Outgoing]
This must be one of the largest, longest running, production grade OAI-PMH operations there is? I say this with some uncertainty, as I don't have a good sense of other initiatives. It would be interesting to hear of other examples which have been in operation for some time ....