As with any new service or site, the
SEO community is quick to exploit it and use it to their benefit.
Wikipedia is probably the lone remaining site that spammers have used to exploit it's authority (value of links fading extremely quickly, if not completely already). They reacted proactively to make sure Google didn't reject them. The addition of the "nofollow" tags on their outgoing links is probably why Google still treats them as a valid source of information. Had they continued to allow spamming, much like Digg, Squidoo and others did for too long, they probably would've been devalued by Google as well.