I have the same problem. I kinda agree with Jay. Typically TabHost, and/or TabActivity utilize a LocalActivityManager that keeps track of embedded Activities or appropriate ContentStrategy component that is displayed within the FrameLayout element. In simple words, this is a typical embedded Activities/ embedded Views layout problem. The Edit Text is on the top-most Activity/View that is taking the touch-screen space, while there's a core Activity that is actually hosting this Activity/View that probably is grabbing the InputMethodService focus and keeping it away from the Edit Text, only for the hard-keyboard scenario. The soft-keyboard just works fine.
One change I did to my Edit Text is to change the InputType as purely decimal. So when the Edit Text gains focus, the soft keyboard shows a numeric key-pad and not the alphabetical qwerty key-pad. I ran it on a Motorla Droid Pro emulator, that I updated in Eclipse Plugins from the Motodev website. Apparently, when I try to enter text from the hard keyboard after having given focus for the Edit Text (and the soft-keyboard is showing a numeric key-pad), after I click 'ALT + 2', the soft-keyboard is reloaded as alphabetic key-pad while the Edit Text loses focus entirely.
Seems like a serious bug to me in the Froyo release, insufficient support for hard-keyboard devices for edit text views in layouts (LinearLayout) that are embedded in other layouts (FrameLayout of a TabHost).