I don't think you need to worry too much. There is noting to suggest there would ever be a jailbreak method for a 3g device specifically, and not for the wifi.
History tells us that the "old" devices are usually the easiest to keep jailbroken in the long term. For example, we know we have the limera1n exploit which cannot be patched on our devices. This at least gives the developers of the jailbreak a way to get through the door and start the process. For the iPad2 they will need to find a new method (or fingers crossed, shatter will work)