Lightroom 1.x has long had synonyms that can be entered as part of the keyword structure. However, synonyms, while they can be exported with images and libraries, cannot be used in searches. This holds true in Lightroom 2.0 beta.
Synonyms help keep the keyword list smaller. In fact, synonyms are a incredible idea for Lightroom to use. However, we need to be able to search on them. As the keyword list gets larger and larger with big libraries, and keywords are deeper in a hierarchy, it becomes all too easy to forget which keywords are used.
In that case, searching for a similar term can make all the difference in finding an image. Please, Adobe, add the ability to search on the synonyms – especially since we already have them input!