Does anyone have a reasonable estimate of how much it took to port say Vice City to mobile? I'm guessing 6 figures, low single digit millions at most, and I wouldn't be surprised if music licensing was a big chunk of that. For iOS, Vice City started as a top 10 app (reported min $47,000/day) and remains as a top 50 app (min $12,000/day). Between iOS and Android, Vice City will likely generate revenue between $5-10 million in it's 1st year, likely on the upper end of that. GTA III also remains in the top 50 on iOS even 2 years years and Vice City being available. So there is definitely a lot of demand for GTA on mobile and a 10x ROI is very attractive.
On the other hand, I believe it was John Carmack that said the concern with established developers doing mobile games is the opportunity cost. Yes it's a big market, it's good mind share to give people access to your game/franchise any time, wherever they are, and the ROI is attractive, but in absolute terms, 10s of millions of dollars isn't a huge financial gain when keeping the focus on consoles to increase chances of a hit there yields 100s of millions or billions in the case of GTA V. I'm sure Rockstar has enough on its plate right now focusing on bringing GTA V to PC and next-gen consoles, expanding GTA Online, and potentially expansions/major story DLC, which would all seem safer and more fruitful (no pun intended) than bringing GTA V to iOS and Android any time soon.
Rockstar has created made-for-handheld tie-ins for each console game except San Andreas. Given the momentum of mobile, unless Sony pays Rockstar for Vita exclusivity or perhaps Microsoft does it for Windows Phone, I wouldn't be surprised if GTA V's spinoff will be made-for-mobile with iOS as the lead platform. Building an engine and designing gameplay specifically for mobile should yield a much better game than just cutting back GTA IV or V to fit on mobile.
On a practical level, the App Store's 2GB file size limit prevents a port of GTA IV or V. Even San Andreas would be problematic. The 2GB limit hasn't changed since the App Store opened, does anyone know if there is a technical reason for this?