This blog is updated daily.
A general description is here.
The new (in 2.9.0) 'stringsAsFactors' argument to expand.grid() was not working: it now does work but has argument default TRUE for backwards compatibility.
expand.grid() gains a 'stringsAsFactors' argument to ask for automatic conversion of character vectors to factors (which happened for many years but was not previously documented). [This did not actually work prior to R 2.9.1.]