Jason Fried of 37signals is making an interesting point, there is no common concise answer to what the terms means.
Three years ago I threw together this definition from various sources:
Information architecture (IA) is primarily about cognition – how people process information and construe relationships between different pieces of information. Information design is primarily about perception – how people translate what they see and hear into knowledge.
These days I tend to believe “I don’t know, but I’m sure the information architect does” and “Information Architect (noun): title invented by web designers desiring a software engineer’s salary.”
Read: An exercise in clarity: Explain “Information Architecture” in 10 words or less