I did include a short "taster" paragraph -- *not* a summary of the story but a vague impression what it was about (Basically, a mention of main character, a hint of problem and of theme. I left plot for the summary). Less than I did when the query letter had to be launched alone, but probably more than I ought to have included on a cover letter.
I was also given a form rejection. I don't think the cover letter was the deciding factor, but it is something to keep in mind.
I always mention what else I've included in the package, be it chapters and synopsis, SASE, chocolate or whatnot. Two reasons for this:
A: If a stack of papers falls and scatters across the room, they know what all to look for.
B: They know I read and followed their guidelines. When I sent the same chapters et. al. to several agents, one wanted the first ten pages, the next the first fifty pages, the next two chapters. Whatever you hear, there isn't one exact standard.
I like Shawn R. Howen's suggestion (Elsewhere on this site) to include a note about what attracted you to that particular publisher/agent; if they had a hand in a favourite book, some professional or particular thing they have done, a brief meeting at a convention. But I've only followed that occasionally.