There is already such an API -- really, just perform the same HTTP
requests that the forms on
http://zxing.org/w/decode.jspx do:
GET
http://zxing.org/w/decode with a URL sent as parameter 'u' and
you'll get back just the text of the barcode
POST
http://zxing.org/w/decode with file contents as parameter 'f' to
also get back just the barcode text.
There is already a native client for iPhone (2D barcodes). I think the
user experience on a such a client are going to be pretty
disappointing, but, sure, have at it.