If that conjures the image of turbines filling up a giant balloon, you're not too far off, although the particular balloon being used isn't elastic. Compressed air works best with pre-existing caves that have proven to be well sealed, like those that natural gas is sometimes found in. Whenever there's excess power coming from the turbines, air is pumped into the cave. When power is needed, heat is applied to make the air expand and blow out.
There are already a couple caves being used that way. One in Germany has been operating since the 1970s, and another in Alabama since the early 1990s. But the technology hasn't gotten too much attention until recently, when some entrepreneurs started working to update it.
Covering the news, the San Jose Mercury points out that the American Wind Energy Association would prefer to skip energy storage for the time being in favor of building cross-country transmission to take wind power where it's needed.
PG&E doesn't agree, because it will take time to build the transmission, and many projects are still up in the air. To that, I'd add another objection: The United States doesn't span enough time zones to make a strong wind at 1am in Ohio or Oklahoma useful anywhere else, especially as we erect ever more turbines. Storage will be useful, if it can be done economically.
Projections have suggested that compressed air might add only a few cents to the price of wind power, which is pretty good considering that the evening wind more or less goes to waste. But PG&E's project would be the first to nail down a figure. At the moment, they're saying 300 megawatts of storage in Kern County, California will cost $356 million, requiring an initial $25 million Federal grant for study.
In its blog post on the idea, PG&E didn't mention who it might work with. But to my knowledge, there's only one significantly-funded startup pursuing air compression. Energy Storage and Power took about $20 million last year from PSEG Energy Holdings, a subsidiary of the New Jersey utility Public Service Electric and Gas Company. That's a pretty good bona fide for working with PG&E.