A different sort of abstraction layer project has been started up and has already seen a few releases - Forage. As mentioned on Rob Young's blog:
Recently I've been working on a search abstraction library for PHP called Forage. The idea is to bring to search what we've had for relational databases for quite a while, abstraction.
On Friday I put up a preview release with three backends; Solr, Xapian and Zend Search Lucene. At the moment it has the bare minimum of features but there will be more soon. In this post I'm going to talk a little about the motivation for the project and then walk through a short example.
He talks about the need for search abstraction (integration and resilience to change) before getting into an example of some code that grabs the data from an RSS feed, passes it in to the Xapian search engine and stores it before looking it over for thier search terms ("yahoo microsoft").
You can download the library if you'd like to try it out for yourself.