Well if you want to save keystrokes, binding the relevant R function to some pre-defined key-strokes, is fast and simple in any of the popular text editors.
Aside from that, I suppose you can always just write a small formatting function to wrap your expression in; so for instance:
fnx = function(x){print(formatC(x, format="d", big.mark=","), quote=F)}
> 567 * 43245
[1] 24519915
> fnx(567*4325)
[1] 2,452,275
R has several utility functions that will do this. I prefer "formatC" because it's a little more flexible than 'format' and 'prettyNum'.
In my function above, i wrapped the formatC call in a call to 'print' in order to remove the quotes (") from the output, which i don't like (i prefer to look at 100,000 rather than "100,000").