Just and FYI..i have been looking for this since I got my EVO. I have terrible service at work and I loved the Roam only feature on my old Winmo phone. Well I have been monitoring a thread over at XDA and finally I found this.
http://www.appbrain.com/app/com.vuvuzela.roamcontrol
I installed it on both my EVO's and it works great...I don't even have to reboot when I change from Home to Roam and back.
FYI I am running Fresh 3.2.0 on both phones and of course you must be rooted. Just thought I'd share