The KDE4 team plans to implement the forward/back buttons in the Dolphin application for KDE 4.7.x, which is scheduled for release 7/23/11. I saw this on the on "Future Plans" page for KDE4 4.7.x here. Needless to say, I was stoked because I have been using those buttons for years now and I've always wanted them to work in the file browsers on Linux. Right on! Linux rocks!