Pierre Moulin has a paper entitled "Information-Theoretic Analysis of Information Hiding" in the March 2003 issue of IEEE Transactions on Information Theory. You can find this paper online here, along with his other related work.

Excerpts from the abstract of the paper: An information-theoretic analysis of information hiding is presented, forming the theoretical basis for design of information-hiding systems. ... The host data set is intentionally corrupted, but in a covert way, designed to be imperceptible to a casual analysis. We formalize and evaluate the hiding capacity ... [which] is the value of a game between the information hider and the attacker. It is shown that many existing information-hiding systems in the literature operate far below capacity.