Linking to Wikipedia. What’s the impact on SEO?

Linking out to relevant sites is never a bad thing and may possibly help your rankings (source).

The only time linking to another site may hurt you is when the site is considered by Google to be part of a bad neighborhood (sites that are created to crosslink themselves and boost their ranking or ranking of a main site). By linking to them you are essentially telling Google you are part of that bad neighborhood . (It may take more then one link to do it by why risk it?)

It doesn't hurt anything. Linking out to authority sites, such as Wikipedia, is a good thing to do. I don't think it helps much, if at all, ranking a site but it does make your site look more 'realistic', especially if you're generating/aggregating content in some way.

I'd make the Wikipedia links nofollow as sort of a quid pro quo since they nofollow all of their external links.