The "priority" attribute in Google Sitemaps
Google states in http://www.google.com/webmasters/sitemaps/docs/en/protocol.html :
The priority of this URL relative to other URLs on your site. Valid values range from 0.0 to 1.0. This value has no effect on your pages compared to pages on other sites, and only lets the search engines know which of your pages you deem most important so they can order the crawl of your pages in the way you would most like.
The default priority of a page is 0.5.
Please note that the priority you assign to a page has no influence on the position of your URLs in a search engine's result pages. Search engines use this information when selecting between URLs on the same site, so you can use this tag to increase the likelihood that your more important pages are present in a search index.
Also, please note that assigning a high priority to all of the URLs on your site will not help you. Since the priority is relative, it is only used to select between URLs on your site; the priority of your pages will not be compared to the priority of pages on other sites.
However, as far as I can tell with my tests, "priority" isn't used at the moment at all. I haven't been able to get a clear answer out of Google on this, it's more of a "if you can provide unique insight, then do it", but "don't do anything sneaky or we will ignore it". My tests have not shown an influence (but it's hard to say what will happen in the future - they're moving very fast!).
Keep in mind that if you do decide to use priority, it should be set with values which are not already a part of Googles algorithm -- values defined by yourself and not by an automation. By using PR or crawl-level you are not giving Google a unique insight into your site - it's something which it knows already. It also makes little sense to set a high priority to deep nodes (or the root) across the band. If you can however say "THIS is a page that is more important than the others but you won't notice that" then use a higher priority for that. Rotating a high priority through your site is likely to be recognized (and perhaps even penalized).
However, since it has not been confirmed (neither by my tests nor by Google itself), I would just ignore the priority setting and keep it out of your Google Sitemap files.