We're just about to start using a DSpace repository for collections data – object metadata, media files and metadata and information record (people, places, events, publications) metadata – for selected records from our Mimsy XG collections management system; and I think an OpenSearch service would make the data a lot more findable and possibly a lot more useable.
I really should write it up properly at some stage, but I'm hoping that our repository will have a use beyond providing an OAI-PMH-compliant data source for partnership projects and our own internal requirements.
For example, other people may query the repository to build applications with our data; or use it as a central index of all the records we've published in digital projects over the years, following links to sites in which the object appears. Or it might enable us to try some semantic web-ish things…
I'd be curious to hear about anyone's experience with DSpace/OAI-PMH or OpenSearch for museum collection data, but I'd particularly love to hear from you if you've used them together.