A web designer friend who dabbles in SEO asked me about a company offering this "service" last June, when it was creating buzz in various marketing newsletters. The company's site looked like total nonsense to me.
More recently, Marcia posted about it in this thread...
If anyone is thinking about or considering "LSI Optimization," there's no such thing.
It's important to understand the concepts behind it, as well as some of the underlying IR concepts, like IDF and KW co-occurrence; but if anyone tries to sell you LSI SEO services, ask them how big the dataset is that they're using.
Google's data for use in any kind of semantic analysis they would use consists of words and (and more importantly) phrases derived from more than 8 billion web pages, and then some - like from documents other than web pages.
Some people throw the term "LSI" around rather loosely. What they are offering as a service is analysis of term co-ocurrence for the top "however many" results on your target searches - and then making sure that the page they are optimizing has a "healthy enough" sub-set of those terms.
This approach can be a bit touchy. If a pages includes too many co-occuring terms, Google might penalize it.