As explained on this introduction to RStudio, the novel IDE can be launched directly on the Web, while running on a server. There could be another short-term solution to use R on an iDevice (or any other smartphone). But is manageable on a larger one (iPad, Galaxy Tab). Typing R code on a small keyboard is quite tedious. Running R directly seems not to be the more sensible way to proceed: smartphones lack so much power for that, you would do it just for fun. But there are some limitations, including bugs in package management.
The steps for the installation through Cydia are described in this R wiki, or this post.
To make a long story short, running natively R on an iDevice (meaning iPhone/iPad) is disabled by its OS, unless it is jailbroken. This thread has been widely discussed on a lot of forums.
