The inability to take screenshots without root or the sdk is a known issue to Android users.
The ability was added in Gingerbread. (I'm not allowed to post links yet but go to phandroid.com and search for a post by Kevin Krause on March 1st, 2011 at 7:35 am for details.) Samsung is the only dev to make use of the option so far (afaik) and only on gingerbread devices.
If you want an app that takes screenshots without root or the SDK, the option is there for you to develop such an app if you don't want to wait for someone else to do it.
Oh, and be sure to post a link to your apk for us when you're done. Thanks.