"So long as" vs. "as long as"

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.

Both the phrases mean during the whole time that.

The Corpus of Contemporary American English reports that the most used phrase is as long as.
as long as is used more in magazines and newspapers than in academic texts; so long as is used more in academic texts than magazines and newspapers.