Creating a blogrolling/link management system

I'm looking at using PHP to create a link/bookmark system similar to blogrolling.com, but for many links both weblog and not. Basically the system would archive 3-500 links, display them hierarchically, etc. Standard stuff that I see a lot of PHP code for. What I am wondering is what is the best approach for detecting "recently changed" links? I could use various files put out by blogger.com or blo.gs, etc. to check sites that use those services, but while being much more efficient than actually checking myself, that would exclude many non blog sites. I only want to indicate sites that were updated in the last 24 hours, so I am thinking that I could just cron a PHP script that checks each site for changes every day. Is there a reliable method for doing this using just the headers returned for a page? Is there a better way?