What about adding zero padding (rjust by "0") by specifying an optional 
second argument? Don't you think it's little nicer to be able to say:

  puts "64bit checksum is #{chk.to_s(16, 16)}"

instead of having to say:

  puts "64bit checksum is #{chk.to_s(16).rjust(16, '0')}"

I believe there's a pretty frequent need for padding when using 
Fixnum#to_s so perhaps this functionality can be added for convenience. 
Besides, this change is backward compatible.

-- 
dave