In the US, you're probably thinking of the NIH's open access policy. The NIH is the largest research agency, with a more than $30 billion dollar budget, so this does cover a lot of US research, but it doesn't cover research from many other agencies.

For much more on the NIH policy, see the links in this article: http://www.earlham.edu/~peters/fos/2007/12/oa-mandate-at-nih...

There has been a concerted (though not yet successful) effort to extend this type of open access mandate to all US federal agencies with budgets over 100 million. See, for example: http://www.arl.org/sparc/advocacy/frpaa/index.shtml

With a lot of work, I believe these efforts are likely to be successful. But there is real opposition, not just from some journal publishers, but also from some politicians. See, for example, the efforts by John Conyers (D-Mi) to roll back the NIH open access policy: http://www.earlham.edu/~peters/fos/newsletter/10-02-08.htm#n...

In the UK, all seven UK Research Councils have similar open access policies. The last one to adopt such a policy was the Physical Sciences and Engineering Council, which I believe is the one relevant to computer science research. That policy was adopted in 2009, I believe, and so we're only starting to see the effects now.