They most likely access whatever app you have set as default for those actions (Dialer & Messaging). This however, will never work with Google Voice, as it is not a Messaging application in the "normal" sense.
Google Voice has a messaging feature, but that's all it is, a feature. Plus isn't not SMS in the traditional sense either; it just sends messages over the internet. In any case it's more of an IM feature than it is a SMS feature.
oh..i didnt realize you can access your default. It appears that any other apps for phone or mms dont allow the long press from long screen to preview the message Handcent has a popup and while the app is nice....i dont like it as much as the stock. I prefer something a little more simple...plus...I use the swipe delete feature a lot.
As well as preview from lockscreen.
Can it be re-written/re-coded to work with GV? Of course, anything is possible. Will it be re-written/re-coded for that purpose? Not likely.
Hey, this may be looong overdue, but there was a dev that made an app that'll emulate the intent that was missing to make GV, as you put it, a "messaging" app. Follow the link, download, and enjoy!
[App] Google Voice Catcher for MIUI (updated 3/12/2011) - xda-developers