never tried Runescape before but it does error on me using icedtea java under 64bit firefox. Have run into this before on a few other sites so I just have 32bit Firfox installed with only Java 1.6.0 and use the firefox-32 tool to start 32bit firefox.
Distribution: RHEL 5.1 with Pieces of this and that.
Kernel 18.104.22.168, KDE 3.5.8 and KDE 4.0 beta, Plus Development src.rpm, ATI fglrx64_7_1_0-8.433-1 rpm with 3D and DRI working.
Acer 5100-5840 with webcam, ati, sdcard reader, sound, atheros based wireless, all working. Only thing not working is the memory stick reader.