BTW Terk, did you ever get ahold of blockchain.info about your banned domain or whatever? I have a contact or two over there I could ping for you if you need it.
Actually I just found that it's an issue with my hosting provider who blocks anonymous proxies and foreign VPS services. They have resolved it just minutes ago so I can now implement the callback.
I also just noticed that blockchain.info has a callback url in their payments API that could be used to populate donation information instead of relying on a cron job, although I think I'd prefer to use both methods in case something goes wrong with the callback, at least I'd have good stats updated every n minutes/hours/days.
Blockchain.info expects the callback to return acknowledgement that notification was processed correctly. If it doesn't get that ack, it will repeat firing the callback at some interval until it gets expected response (or until it tries so many times that it puts your domain into a blacklist). So in theory that should work well even if you have temporary problems with your website.
I think not everyone has access to cron in their hosting environment and not everyone knows how to put something in cron. I think that since Blockchain.info repeats firing the callback if there's something wrong on your side, it should probably be enough to rely just on that callback. I may add additional checking by cron if there will be reports that stats are not always reliable.