JohnMu from Google posted in a Google Webmaster Help thread that Google typically crawls a site's robots.txt file on a daily basis. This is the first time (at least that I can remember) I have seen a Googler make a statement on the crawl frequency of robots.txt files.
We usually only check the robots.txt file once a day for most sites, so I assume you were just still seeing the version that we fetched yesterday.
I have not validated this with my sites log files, but that is not the point. The point is a Googler said, on a general level, how often Googlebot will refetch a site's robots.txt file.
Forum discussion at Google Webmaster Help.