According to some websites, Take2 was pretty flippant about GTAV Single Player Story DLC Content:
*edit after linking to banned website?*
Oh, I guess that website is a banned one...? I had never heard of it before. Anyway, on Flipboard I read an article that on the investor call today, when asked specifically about GTAV single player story DLC, Take2 said that they had not made any promises for single player DLC. When asked by someone on the call that they thought the publisher had mentioned something about it before, the Take2 person answered "we remember differently."
Doesn't sound good. Maybe they'll blow something up at E3, but you can't imagine why they wouldn't hint at it on the call... Or pull a "We'll have more information about that in the future" in some non-committal way.