Upload pb: works with arduino ide (not "with programmer") , not with avrdude

Hello all,
I've just received my official Arduino2560. I'm on Linux Fedora14, downloaded arduino 1.0, I can upload "Blink" just fine by hitting "upload", the loopback test works fine.

I don't have a programmer (awaiting one). So I upload using just my USB cable.

I've downloaded an interesting project (grbl) that is not in Arduino IDE but comes with a makefile and uploads its .hex with avrdude. And no matter what options I give avrdude, I get "programmer is not responding" (see below for the exact messages). I tried all -c options that seemed relevant. Tried reset as well. Tried 57600. 9600, 115200 bauds.

I tried to do "shift-upload" in the Arduino IDE hoping to see what command line it calls but then I get the same errors, apparently the uploading method is not the same. I googled high and low including searching in this forum, no joy.

So is there a command line that does exactly what the Arduino IDE does when it uploads a .hex (not shift-upload) ?

Thanks for any hints

/home/jmc/Downloads/arduino-1.0/hardware/tools/avrdude -c Arduino -P /dev/ttyACM0 -B 115200 -vvvv  -p atmega2560 -F -C /home/jmc/Downloads/arduino-1.0/hardware/tools/avrdude.conf -U flash:w:grbl.hex:i

avrdude: Version 5.11, compiled on Sep  7 2011 at 19:34:16
         Copyright (c) 2000-2005 Brian Dean, http://www.bdmicro.com/
         Copyright (c) 2007-2009 Joerg Wunsch

         System wide configuration file is "/home/jmc/Downloads/arduino-1.0/hardware/tools/avrdude.conf"
         User configuration file is "/root/.avrduderc"
         User configuration file does not exist or is not a regular file, skipping

         Using Port                    : /dev/ttyACM0
         Using Programmer              : Arduino
         Setting bit clk period        : 115200.0
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: Send: 0 [30]   [20] 
avrdude: ser_recv(): programmer is not responding
avrdude: stk500_recv(): programmer is not responding

avrdude done.  Thank you.