I'm not aware of any special relationship or consideration google gives to SO, if that's what you mean, and I'd be very surprised to hear there is one. There's nowhere at Google you can go and say, "I want to use Google for my site's home page, hook me up." Instead, think about it from a search-engine's perspective.
If I were to design a search engine spider, I would have some default frequency that my spider returns to a site to re-index it. When it re-indexes the site, it will check to see if the content has changed. If it has, the spider might want to increase the frequency and index the site more often. If the site hasn't changed, it might decrease the frequency.
The problem here is that indexing a lot of sites over and over has a cost. Just because content changes a lot doesn't mean anyone cares about it. So I might also have a score for each site that says how important it is. Think "PageRank". Now when my spider finds a site that changes a lot it might not increase the frequency beyond what the score for the site justifies.
So with Stack Overflow we have a site with content that changes frequently and that has managed to earn a good PageRank, the main requirements for frequent indexing. Additionally, StackOverflow does a few things to make it easier on google's spider to find the new pages. Probably the most important of these to SO is the xml sitemap. I know they put a lot of work into getting that right.
Some other features that help with searching and rank are using
rel="canonical" to avoid splitting rank across different links to the same content and using search-engined optimized urls to put more keywords into the url. You might find the SEO section in this answer helpful.