Security wise it's a good default. Human wise, probably not as it's likely to be the first big-surprise for new users no matter how good the docs, especially for those new to linux. Having said that, I think Josh mooted the idea of a Quick Start Guide -- this could go a long way towards avoiding the gotcha.
As it stands, I'd reluctantly vote for opt-in (and hope that my neighbours aren't deploying CMS alpha 1 bad boy).
Either way, it would be a massive improvement if the mechanism alerted on fail. At the moment there's a real risk updates and other functionality will silently fail. Worse, we wouldn't be aware that potentially malicious activity is in play. I'd rather have 10,000 emails to root than not know.
It may be a pain to support but for me it's warm-fuzzy functionality - even as a silent killer.