R&D mode gives greater access to the device. Among other things, it allows one to gain full root access in Xterm on the 770 by typing "sudo gainroot" in that mode. Thus, many people have been switching their devices into R&D mode, modifying the gainroot script so you no longer need R&D mode to become root and then turning R&D mode back off.

http://maemo.org/maemowiki/HowDoiBecomeRoot

You'd know if you were in R&D mode because you would have had to download and run the Flasher program for Linux in order to enable that mode. You can easily see if a device is in R&D mode - there is an extra icon under the Nokia logo when booting and it shows a lot of info on that screen that isn't normally there, like kernel version etc.