There should be a much easier way to do this. From my understand with people who have Samsung Vibrants running Froyo and people with the leaked build of EB01 Froyo on the Fascinate, there are a few of things you can try doing.
1. Hold the Power and Back key at the same time, this works on the Fascinate running leaked Official Froyo.
2. Many Vibrant users reported "accidentally" taking screenshots they are not sure what they pressed, but there is a software out there for the vibrant that allows you to shake the phone and it produces a screenshot. I believe it only works on Galaxy S models and no other Android phone. (Source: http://forums.t-mobile.com/t5/Samsung-Vibrant/Samsung-response-Screen-capture-other-features/m-p/699109
3. On the international Version of the Galaxy S (GT-i9000) they hold the back key then the home key to take a screenshot (apparently different from the Fascinate)
There is a way to do it on all Galaxy S models w/o rooting it. It's already known on the Fascinate and the international version. People are still trying to figure it out on the Vibrant. I'm sure the Captivate and Epic are capable of it as well. As stated previously, the only Android phones with the capability of screenshotting are the Galaxy S phones w/o root access.
BTW: Check out this app, it only works for Galaxy S devices and Galaxy Tabshttp://www.appbrain.com/app/screen-capture-shortcut-free/jp.tomorrowkey.android.screencaptureshortcutfree