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I include an alt tag on every image on my website.  I use the alt tags not for keywords, but a clear description of what the image is trying to portray. The description usually includes a the keyword, but also includes other descriptors as well.

Bottom line...They probably don't care as much weight but that must certainly carry some. To echo TextMarketing, my hunch is that no-follow editorial links from high-authority sites such as Wikipedia are a sign of trustworthiness.

There doesn't seem to be a real definitive answer to this question. back in 2008 Google talks about exploring new and improved ways to crawl pages including some HTML forms. It's been almost 5 years since then so I'm sure they're doing more than before. I would still however include some text links on pages if you're looking to send web crawlers there. Also make sure that you have a solid sitemap to help the crawlers find your content on these pages.