It should be noted that Rebol comes with an arbitrary precision (bignum) arithmetic option via it's money! datatype - http://www.rebol.net/wiki/Money

  >> .1 + .2 + .3 
  == 0.6000000000000001
  
  >> type? 0.1    
  == decimal!
  
  >> $.1 + $.2 + $.3
  == $0.6
  
  >> type? $0.1
  == money!

Red will most likely get this soon but perhaps with a different nomenclature (Red as already renamed decimal! to float!).