I've just uploaded my first app on the market. It all went and looks well. I tried a few keywords to search for it, words that I also have in my description AND promo text, but some words don't find my app, some do.
How does the keyword strategy work for an app on the market, I couldn't find no documentation on it.
I think the only place this is documented is the Publishing Your Applications page in the Dev Guide. At the bottom of this page you'll find the meaning of various Market URLs, including:
market://search?q=<substring>
Searches all public fields (application title, developer name, and application description) for all applications. Returns exact and partial matches.
Since that URL brings up the Market's "Search" activity with the substring/query in the search field, I think it's a reasonable assumption that the behaviour is intended to be the same when a user initiates a search manually.
I note that promo text is not mentioned as a searchable field, but why a search would not find something that's in the description, I don't know.