A great blend of architecture and design
The best way to think about the difference between an information architect and a designer is to think about the difference between a building's architect and an interior designer.
The architect primarily cares about structure, flow, and such fundamentals as placement of plumbing and electrical systems. If the architect doesn't do his job, then the building might collapse or fail to meet the needs of the people using or living in the building. For example, there may not be enough bedrooms.
Interior designers, on the other hand, care about colour, placement, and style of furnishings; textures; surfaces; and sensory appeal. They may be trying to provide a certain look to rooms, such as Mediterranean or Spanish, or making sure that colours and styles are themed throughout the structure.
This is not to say that either job is easier or harder; they're just different. Obviously, there will always be a little overlap (for example, the architect does care about visual appeal, and the designer does care about flow and access), but in general the two disciplines complement each other. You'd never ask an interior designer to architect a house, and you probably wouldn't go with an architect's opinion of a colour scheme for the walls of your living space.
Shifting gears to Web development, the parallels hold. The information architect generally doesn't have much training in identity design, colors, layout, and certain forms of visual communication -- this is the expertise of the designer. However, the information architect is usually someone with a background in categorization, XML, content creation and organization, interaction design, and navigation design. Their expertise is in information structures, and the rest of this article will be devoted to when, where, and how this expertise is applied.