I hope you're ready to go city hopping around wine country because there just so happens to be a spot in each city that's famous for its bubbles! Here they are:
Domaine Carneros -- Definitely the most popular place for bubbles in all of wine country, not only because it has a fantastic selection of bubbly, but also for its sweeping views of Napa and its vineyards. Be prepared to walk up some stairs to be perched up in the valley! Great to grab a table on its patio and a flight of bubbly on a sunny day.
Domaine Chandon -- Another great destination for bubbles and beautiful views. Opened in the early 70s, this was actually the first French-owned sparkling wine winery in California.
Schramsberg Vineyards -- This one's more expensive than the others, but along with a tasting, you also get their famous cave tour for $45 (at least last I checked in 2012) and it's totally worth it
Gloria Ferrer Caves and Vineyards -- I just threw in this one as an afterthought for if you have time. They do have good sparkling wine. However, they do not do tastings; you have to get either a glass or a bottle.