OK, here's a variation on the original question. I'm working with a site that due to a site malfunction had wrong URLs in place for long enough that they got spidered. The malfunction was corrected, the correct URLs were put back in place, and the wrong ones 301ed to the correct locations.
Oddly, though, Google is spidering the new pages but still has the old pages in its index. At one point, I even tried putting some prominent links to a portion of the old URLs, thinking that Google would follow the link, discover the 301, and drop the old URL. Hasn't happened, though, despite otherwise aggressive spidering. The old pages live on like zombies in Google's index.