Different people and organizations have different perspectives on this. For me an interaction designer bridges the gap between an information architect and a visual designer. Information Architect technically should look into information organization and content labellings, tagging, etc.
However, how the user interacts with the information is decided by the interaction designer – whether a cascading menu is needed or two level of tabs. Or for e.g. where and how to place controls in an email client. Also, when the world is rapidly moving towards RIA there is a lot of scope to add animation and other effects on the interface. RIAs have changed the way we have been interacting with applications and websites and certainly soon will be adopted by everyone. In that scenario the interaction designer would become critical in deciding on the animated effects not to complicate things but to make them simpler and interesting for the users. In a single line I would say that Interaction design is microscopic design, whereas information architecture is macroscopic design. Interaction design exists at page level and rarely influence the information organization. On the other hand, information architecture exists at site level and influences interaction design.
Interaction designer generally do not interact with users. They do not need to sweat out themselves to think about information organization and content. But ideally should not do visual design. The boundaries are really fuzzy here and it is difficult to make a distinction.
In most of the organizations, either an information architect (usability specialist) or visual designer steps into the shoes of interaction designer and does the job.
