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
adb shell
chmod 755 /data/local/tmp/rageagainstthecage-arm5.bin
cd /data/local/tmp

exit out of the adb shell, and type adb shell again. this time it should drop you to # instead of $.

if it goes back to $, run the exploit again,
cd /data/local/tmp
and exit out of adb and then adb shell again to see if its # .

if it doesnt work the first time, repeat 4-5 times and let us know. it should work!

To be clear, the file you'll need is HERE. Are you so cool that you already have an Epic 4G? Want to tell us if this method worked for you? Tell us about it!

Source: SDX-developers via AndroidCentral