Amazing how certain people want to spread FUD (fear, uncertainty, doubt).
First, why are we even talking about the possibility of Obihai going out of business? There is no indication whatsoever that this might happen anytime in the near future.
Second, even if they did, why would they not give people some warning first, and instructions for setting their devices to do manual configuration rather than using the OBiTALK portal? It's not as though there's some reason they'd have to slink away under cover of darkness!
Third, your Obihai device would retain its existing configuration and keep working with whatever service providers you have it set up with. The only two things you would not be able to do is to use the OBiTALK network to place and receive calls (note that has NOTHING to do with Google Voice), and use the OBiTALK portal to configure your devices. You would still be able to manually configure your device by browsing to the device's IP address (which you can hear by dialing ***1) but you would need the admin password to log in (which is visible in the OBiTALK portal). If you didn't have the admin password then presumably you could factory reset the device and start from scratch.
Fourth, if you are really concerned such a thing might happen (why?), go into the OBiTALK portal and write down or otherwise save the admin password you see displayed for each of your devices. That way you will have it in case the unthinkable happens.
And finally, even if this site went away, I'm sure some information would be available on other sites. Examples might be
BroadbandReports.com, Voxilla, the Michigan Telephone blog, the PBX in a Flash forum, etc. There are many sites that cover VoIP and there's always the possibility that an Obihai device forum might be started on some other site. Combine that with the information available via the aforementioned Wayback Machine and I thing that anyone with even a modicum of technical knowledge would be able to make these devices work for the foreseeable future.
I do sort of wish someone would reverse-engineer the firmware (as has been done for certain routers, resulting in alternative firmware such as DD-WRT, Tomato, etc.) so that we could get features that Obihai seems to be unwilling to provide, such as Caller ID name lookup on incoming Google Voice calls. But the existing firmware will still be quite usable even in the unlikely event that something happens to Obihai.