I don't know that the distinction between these two is formality. I think that "so long as" indicates a logical predication, and "as long as" indicates a specific time interval. Take your two examples.
- So long as Google Voice allows free long distance in North America, I will use it.
I think this could be paraphrased as
- My use of Google Voice is conditional upon free long distance calls in North America.
Your second example was
- As long as Google Voice allows free long distance in North America, I will use it.
This could be paraphrased as
- For the duration of time that Google Voice allows free long distance in North America, I will be using it.
Thinking about it, I think that both "so long as" and "as long as" are used for the conditional reading, but only "as long as" can be used for the duration reading.