Hi-

Just a couple of preliminary things-- I think if you 'binarize' something, by definition the only values can be 0 or 1 (i..e, you can't binarize and have values 0,1,2,3,4,5). Also, generally a 'mask' is specifically *nonzero* numbers, so I don't believe it is accurate to say "I have a mask of 0/1" values. Instead, you could have a mask of 1s, surrounded by a sea of nonmasked area of value 0.

To your question, a simple way to make a binarized mask (only values of ones in the mask, zero elsewhere) from a file with values >=0 would be the following:
$ 3dcalc -a FILENAME -expr 'step(a)' -prefix NEWMASK
The 'step()' function creates a dataset with value 1 wherever a voxel in 'a' >0, and 0 where a voxel in 'a' is zero. That sounds like what you want?

--pt