There are certain things that hot weather makes me crave... salads, watermelon, banh xeo, ice cream... And after a trip last summer to Japan, I can now add cold soba to that list! I absolutely loved the noodle stands there, and cold soba seems like such a staple in Japan, but I rarely see it offered at Japanese restaurants in the states. I know Mifune is one option; can anyone recommend any others?
Hotei on 9th just off of Irving and across the street from Ebisu has an excellent selection of Soba dishes.
For something likely no other place in the Bay Area can match at this, try Mikaku on Grant near SF Chinatown. There have been (off the board) reports of teuchi soba (i.e. hand made fresh on the premise) on their specials board ($8.50) and be sure to ask for sobayu at the end of the meal (the dregs and byproducts from the soba making process, in the form of liquid, apparently very nutritious, much like okara from the tofu making process which you can also get at San Jose Tofu). Though call ahead, the fresh soba might be a dinner only offering.
I find all the dishes at Mifune pretty mediocre. It seems to be parennially packed, probably thanks to its great loation.