You might try this:
http://forum.xda-developers.com/showthread.php?t=696073
Its NOT roaming however it is....Let me explain.
Sprint and Alltel (now Verizon) have a roaming agreement. If you use Alltel's towers it is the same as if you used Sprint towers. In other words your not in roaming but you still use Verizon 3G. Verizon 's 3G is faster than Sprint's 3G. You will not be charged for roaming because technically your not roaming. This may help people who have bad Sprint 3G coverage but have good Verizon 3G coverage. If it makes you feel comfortable you may turn off Allow roaming in the phones settings.
I do believe you need to be rooted to do this. Which is also very easy to do now just go to this site on your Evo:
http://unrevoked.com/m/
Tap download...and install. Thats it. your rooted.