You should know that dplyr, plyr's replacement is already fairly stable and worth using. For basic tasks, it's as fast or faster than data.table in my experience, with the caveat that it is more likely to copy for some methods than data.table which is very strict about this.
And being able to combine it with an RDBMS means that you can potentially do plyr-ish things on datasets that can't fit in memory. I think the SQL generated by dplyr tries to be smart / efficient too.
