in frontend mode status is printed in json format and should be parsed
one line at a time, example line:
 {"duration": 28.953000, "position": 1.76, "audio_kbps":  59, "video_kbps": 292, "remaining": 15.45}
last line indicates result, if all went well you get:
 {"result": "ok"}
if input could not be parsed at all you get:
 {"result": "file does not exist or has unknown data format."}

example in python to parse ffmpeg2theora in frontend mode:

'''
import os
import simplejson

cmd = "ffmpeg2theora  --frontend ..."
f_stdout, f_stdin, f_stderr = os.popen3(cmd)
info = {}
line = f_stdout.readline().strip()
while line:
    try:
        info = simplejson.loads(line)

        #do something with info dict here
    except:
        pass
    line = f_stdout.readline().strip()
'''