This paper presents technical foundation, roadmap and initial results of the IDIOM project (Information Diffusion across Interactive Online Media). Information spreads rapidly across Web sites, Web logs and online forums. IDIOM tracks this process and compares it to direct communication through electronic mail and instant messaging. Linguists define "idiom" as an expression whose meaning is different from the literal meanings of its component words. Similarly, the study of information diffusion promises insights that cannot be inferred from individual network elements. Similar projects often focus on particular media, or neglect important aspects of the human language. IDIOM addresses these gaps to reveal fundamental mechanisms of information diffusion across media with distinct interactive characteristics.