One of the most popular hashtags on Twitter today is #iagaymarriage, which people are using to comment on this morning’s Iowa Supreme Court decision legalizing same-sex marriage. That’s fairly standard for big, breaking news, but this isn’t: The hashtag was created by Iowa’s largest newspaper, The Des Moines Register.

In anticipation of the ruling, the Register’s social media editor, Cavan Reagan Reichmann, sent out a message yesterday afternoon from the newspaper’s Twitter account, which has more than 2,200 followers:

Speaking over the phone from a busy Register newsroom this morning, digital editor Chris Snider told me, “We thought that we probably had enough clout that we could just create our own hashtag and hope it spreads.” It did. After the decision came down this morning, even competitors like The Gazette in Cedar Rapids were tagging their tweets with #iagaymarriage.

The hashtag’s proliferation has allowed the Register to collect reactions to the ruling on a dedicated page of the site, as well as in the sidebar of its news stories. Snider said that if the hashtag hadn’t caught on, their “backup plan” was a Twitter search for terms like “Iowa” and “gay marriage,” which could have produced a similar result.

The Register’s creation of #iagaymarriage is notable because while Twitter is all the rage among news organizations, most still treat the platform as something to be observed — as in, here’s what people are saying about this topic on Twitter. The Register, on the other hand, could rightfully boast: Here’s the conversation we helped organize on Twitter. It’s the man-on-the-street quote taken to a massive scale.

When devastating floods hit the Northwest earlier this year, Elaine Helm, new media editor at The Herald in Everett, Washington, helped establish the #waflood hashtag that spread like, say, the California wildfires hashtag (#ocfire) last year. In both cases, news organizations used the tags to gather news and reactions.

This is the first time that the Register has collected tweets about a breaking news story. Last week the newspaper launched a great landing page to follow its reporters — and some readers — on Twitter.

Snider, who has previously worked at The St. Louis Post-Dispatch and Baltimore Sun, has a great checklist that he follows for big news stories like this morning’s ruling on gay marriage. (It’s literally a checklist that you can download as a PDF.) Here are Snider’s recommendations for promoting a breaking news story:

— Post to Twitter
— Send breaking news text alert
— Send breaking news e-mail alert
— Post to Facebook
— Send update to Facebook fans
— Send a message to MySpace friends
— Create a widget so others can add news to their site
— Send to Drudge
— Post to Reddit
— Post to Digg
— Send to Fark
— Post to del.icio.us
— Post to Newsvine

Speaking of the Drudge Report, when I spoke to Snider, shortly after 10 a.m. in Des Moines, the Register’s traffic was elevated but “not crazy,” he said. They had about 215,000 pageviews at that point, compared to 160,000 by mid-morning on a typical day. But while we were on the phone, Drudge linked to the Register’s news story about the ruling at the top of his highly trafficked site. Snider said, “Well, now this thing is about to go through the roof.”

UPDATE, 3:39 p.m.: Snider tells me that the Register has seen 180,000 unique visitors already today. (That’s different than the pageviews metric cited above.) “We consider 80-100,000 uniques to be a pretty good day,” he writes in an email.