That was quick - the Sprint's Epic 4G has already been rooted by the crew over at SDX-devs. In other words, now you can root that phone you don't have yet. It's a method requiring adb (Android Device Bridge) commands and thus desktop connectivity, but a one-click solution will probably emerge sooner rather than later. Note: it says EVO root, but the method works on this phone as well, don't let the filename fool you. Instructions below:
download this file
run these commands thru adb
adb push c:\downloads\rageagainstthecage-arm5.bin /data/local/tmp
chmod 755 /data/local/tmp/rageagainstthecage-arm5.bin
exit out of the adb shell, and type adb shell again.