I like the question.
My experience is you have what I call top down business consultancies like Bain, BCG, McKinsey, etc. They are the ones who decides the overall business strategy and then have tactical services like branding and design. As a rule of thumb, these types of firms are doing Fortune 500 strategy/branding/design. They are the ones with a track record to lower risk for companies that will spend 9 figures on build-outs, packaging etc.
Then there are what I call bottom up design consultancies like any that have been listed in this thread. They primarily do the tactical design but then offer strategy as a secondary service. While they will have some Fortune 500 clients, the medium- and small-size companies will be their bread and butter as their risk is lower and cannot afford the big boys.
On a side-note, http://www.555.com/ , is one of those bottom up consultancies. Kind of a private dream job for me. They mostly do architecture/design with one-offs or small chains but they did do Armani Exchange a while back.