Did you know there's a term for the foam that lingers on the side of your beer glass? It's called Brussels Lace, and supposedly it's a measure of the beer's freshness.
Some beers leave more lace than others. A good german Pilsener, like Dinkel Acker CD Pils will leave clingy rings down the whole length of the glass, whereas no pale or brown ale will leave any. So take Brussels Lace at face value; It's like Anna Kournikova on the tennis court: there for the visual appeal alone.