I've told LukePH and hybridOL (and a number of #irrlicht lurkers) already, but as I noticed probably jamming bug reports into the irc channel is probably not the best thing -> here's a bug report.
Basically, irrlicht is not strict aliasing safe, according to gcc, and by default with 4.2x and above versions is set to compile with strict aliasing with O1 optimisation and above. This causes some problems with collisions and various other things, if you use the Makefile provided - e.g. make NDEBUG=1
Two possible solutions -> take the bug related stuff with my diff (i've sent to LukePH) and merge them in, or set the makefile to explicitly disable strict aliasing.
e.g. -fnostrictaliasing
Cheers!