How do I know if an article is Open Access?

"One of my pet projects is to build a "Universal Article Reader" for the iPad (or similar mobile device), so that a reader can seemlessly move between articles from different publishers, follow up citations, and get more information on entities mentioned in those articles (e.g., species, molecules, localities, etc.). I've made various toys towards this, the latest being a HTML5 clone of Nature's iPhone app.

One impediment to this is knowing whether an article is Open Access, and if so, what representations are available (i.e., PDF, HTML, XML). Ideally, the "Universal Article Reader" would be able to look at the web page for an article, determine whether it can extract and redisplay the text (i.e., is the article Open Access) and if so, can it, for example, grab the article in XML and reformat it. Some journals are entirely Open Access, so for these journals the first problem (is it Open Access?) is trivial, but a large number of journals have a mixed publishing model, some articles are Open Access, some aren't. One thing publishers could do that would be helpful would be to specify the access status of an article in a consistent manner. Here's a quick survey at how things stand at the moment."