Within our local education's I.T system, all websites served via HTTPS are blocked, except a select few websites which are 'authorized'.
This results in some websites being unable to function properly (such as Google's jQuery libraries, which are served via HTTPS).
I also remember reading an article a couple of years ago which said that HTTPS can cause problems with censor filters/proxies, as HTTPS connections cannot be intercepted & the page contents searched for blocked content (I haven't seen this behavior in many filters, but one such software that does this is censornet).
Disregarding educational use, I have also found that a few shared WiFi hotspots also engage in this behavior (of which are unfiltered for the most part). For example I was travelling via Gatwick Airport (U.K) last year & all HTTPS sites were blocked on the official 'Gatwick Airport WiFi' network.
Other than for filtering purposes, is there any reason to block HTTPS? HTTPS is paramount for websites that deal with private information (passwords, emails, etc), so I can't really see a good reason for blocking HTTPS.