The next release of open-source R, codenamed "Roasted Marshmallows", is scheduled to be released on June 22, according to this announcement on the r-announce mailing list. Don't expect too many changes in this update: despite the fact that "there have been very few issues with 2.15.0 ... some people may be waiting superstitiously for a .1 release". This will probably be the final patch before 2.16, due sometime next year now that R is in an annual release cycle. (Correction: R Core member Duncan Murdoch notes in the comments that there will, in fact, be at least one more patch before 2.16.)
Revolution R Enterprise, meanwhile, will be updated to the final patchlevel of the 2.14 series (R version 2.14.2) when Revolution R Enterprise 6 is released in June.
r-announce mailing list: Release plans: R-2.15.1 on June 22