Google Page Rank forum discussing tips and optimization techniques for both white hats and black hats to improve a site's PR. Talk about PageRank flux, sudden drops, and the biggest contributors to it.
No, Wikipedia makes no difference to PR of linked pages, either positive or negative.
This is intentional on Google's part - they have announced this (see somewhere in the Google blog). This is to discourage low quality links from being added to Wikipedia articles.
Since WikiPedia is a public space, which can be spammed, WikiPedia makes all outgoing links "nofollowed" meaning there is no "PageRank juice" which flows outward.
As long as your link profile is solid, a link can't hurt you. If it could, you could focus on bad links to your competitor's sites.