According to Microsoft (via IGN) downloadable content for Grand Theft Auto IV is coming this fall. Here, word-for-word, is what M'Soft said:
"In autumn there will be news from Liberty City; the first episodes of GTA IV will be available for download exclusively on Xbox LIVE, underlining Xbox 360's leadership role in the field of digital entertainment."
Tantalizingly lacking in details, eh, gamers? Like, what kind of content? What will it cost? Etc. We'll let you know as soon as we know, of course.