In spite of the efficiency of contemporary armor, shields were used more often in armored combat on foot than you might suppose, but not necessarily in the way you might expect, particularly if your expectations are conditioned by the SCA’s standard recreation of medieval combat. Their use is recorded in this combat of seven against seven in 1402 and Habourdin vs. de Bearn in 1449. Techniques for their use are illustrated in both the Codex Wallerstein and the Gladiatoria Fechtbuch.
In those sources they were primarily used as a defense against thrown spear before the champions came to close quarters. They were also strapped or handled so they could give partial protection while the user wielded a spear or sword for two handed thrusts. Once no longer useful for these purposes the shield would be either thrown at an opponent or simply discarded.
I am currently working on a pair of the sort depicted in the Codex Wallestein, and will make every effort to have them finished in time for Pennsic 2009 and the Company of St. Michael’s recreation of the pas de la Belle Pelerine. The Habourdin vs de Bearn combat was a continuation of the historical pas de la Belle Pelerine, granted after de Bearn fell ill on route to the pas and arrived after the appointed time had expired.