Hi!
I'm planning to develop an small application involving several Arduinos and other hardware in a RS-485 network. While speaking with my office colleague, I realised that there is a chance to enter into bootloader mode during start-up (also power-up or any sort of reset) if receiving the right message.
What do you recommend me to avoid this unwished situation?
I haven't found anything appart from the Bootloader spec.
Thanks in advance ! 
/me
It is possible to compile a sketch in the Arduino IDE and then burn the resulting Intel Hex file into a target Arduino board using a hardware programmer. Not only do you eliminate the bootloader you gain the extra memory size that the bootloader would have taken.
Specific steps to perform this might vary with programmer type used and Arduino IDE version used, but here is a link that talks about it.
I recommend the TinyUSB programmer, it is supported by the Arduino IDE and works great for me so far:
Here is similar one that is preassembled:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170376999470&ssPageName=STRK:MEWAX:IT
Lefty
Take a look at Ladyada's bootloader:
http://www.ladyada.net/library/arduino/bootloader.html
It goes straight to the sketch unless the reset button has been pressed, eliminating the problem you mentioned.
Take note that the bootloader runs at 19200, rather than 57600, so you'll need to make a new entry in arduino/hardware/boards.txt.
Justin