I think big telco's have to diversify, not just because of google voice, but now their only leg up on the cable industry is gone. The cable industry for years now have had a much better high speed solution (although they have not always fully pushed the envelope). At&t just could not compete with the speeds that cable could offer, so they competed on price.
Now the cable companies are stepping directly on their feet with voice products of their own. Add that with the fact that VOIP is becoming more popular, and that many people are dropping land lines for cell phones, the days of traditional land line phones are numbered.
The only way AT&T can compete is diversifying, although I think they are bringing in U-Verse to slow, and to late. Cell phones were a obvious choice for them early on, and partnering is Dish was a good choice (although because of the preconceptions about dish based tv, it has not been a huge success), now they really need to be able to compete with high internet speeds and a direct line cable.
In the future I believe AT&T will not have much of a traditionally structures telco, but rather a similar setup as cable, or a better one.
So they have a rough road, but they can make it easier by embracing the change, rather than trying to fight it.
I think Google voice may offer a little heat, but I think the biggest thorn in AT&T's side is going to be the cable companies.
Google Voice or any thing similar aren't a threat to telcos, as no VOIP can work without the network/infrastructure that telcos have implemented. So whether you are paying or not, telcos are still getting their revenue.
Also what percentage of user base is there who is ready to keep their computers on all the time to use google voice and you have to have an existing phone to use google voice.
Not until they can have dialtone reliability the way incumbents have.
They dabble at a whole bunch of things that just don't have the reliability that it takes to get to the next stage. Apps, Docs and Mail for example have had security and availability problems regularly. The degree of availability and accountability suitable for business use just is not there. They might be fine for those that don't care. But, for those that do care, it would be the source of too many nightmares.
They've managed to convince a whole bunch of people that they are invincible. The only thing invincible about them is their PR machine.