This is a very interesting question -- R is indeed full of idioms, and the situation is made even more difficult by the fact that there are many idioms for data analysis, in addition to the more general programming ones. Combined with R's expressiveness and its penchant for violating the principle of least surprise, this often makes the learning curve a bit steeper than one would like.
Personally, I picked up most of what I know by reading help, reading various tutorials and tip collections, and occasionally looking at source code of built-in functions. R FAQ has useful tidbits to start with. Revolution Computing has links to good resources, particularly for programmers. Also, I found Howard Seltman's collection of tips and links to be useful; I would bet that links on that page would cover most useful R idioms, but I am curious to see what else is out there.