As an example, we may get 20-30 valid registrations per day, but in the last month there have been a couple days (maybe xrumer broke our captcha, etc.) where it went up to 400+.
Sure, we can deal with them after the fact, but we don't discover them until they start to post, etc.
This could be as simple as a small cron job which "inform admins when daily registrations are over X", X could be a number or multiple (based on average of last week, etc.).....
Sample enclosed shows that registrations are usually within a range when things are normal.