Which local restaurants cellar wines until they're ready to drink, or at least one way or another have a good selection of mature wines on their lists?
The first place that comes to mind is Rubicon:
Gary Danko's got some remarkable old vintages, some at even more remarkable prices:
Chez Panisse has some mature wines, e.g. of their six California cabs, four are from 1993 to 2000:
Oliveto's gotten a lot better on this score, e.g. 14 of 17 Tuscan reds are 2000 or earlier: