Looking at the code it looks pretty easy to circumvent: don't use firebug, have the devtools open when loading the page or have them open undocked by default. The detection method for non-firebug devtools is pretty silly: it checks the window size.
Even so, I'd appreciate it if the browser vendors took additional steps to disable the detection of devtools, personally. I don't think they should be detectable under any circumstances, if that's possible to implement.
