I had though about this a couple times. But nearly any post I would subscribe to I am also making a post in. Since I use viigo to follow all the new posts, if there is one I want to followup on and don't need to reply to I save it in viigo. Alternatively you can email the URL to your pc email to subscribe using the forum on the pc, or add a bookmark in the BB browser, depending on how long you want to follow the thread.
Posted from my CrackBerry at wapforums.crackberry.com
sorry