It’s finally happened: a major publishing house has announced it will abandon DRM protection for its ebooks. Yesterday afternoon Macmillan sci-fi imprint Tor/Forge announced in a release on Tor’s website that “by early July 2012, their entire list of e-books will be available DRM-free.”

“Our authors and readers have been asking for this for a long time,” said president and publisher Tom Doherty. “They’re a technically sophisticated bunch, and DRM is a constant annoyance to them. It prevents them from using legitimately-purchased e-books in perfectly legal ways, like moving them from one kind of e-reader to another.”

The brief announcement said the company’s ebooks will continue to be available at all the places selling them now, as well as additional retailers “that sell only DRM-free books.”

On the one hand, not a major announcement. After all, it’s long been clear that DRM doesn’t do much to slow down piracy, the ostensible reason for its existence.

On the other hand, it’s a major announcement in the ongoing battle against the Amazon ebook monopoly.

As it’s the retailers who install DRM on ebooks, the Tor/Forge announcement signifies that the imprint — and note that it’s just the imprint, not the Macmillan mothership — has instructed all of its accounts — most notably Amazon.com — not to install DRM on its ebooks. It’s hard to imagine that went down well in some quarters, or that there wasn’t a fight about it, considering that retailers such as Amazon and Barnes & Noble use DRM to render the ebooks they sell into virtually proprietary software — i.e., readable only on their own devices, and unreadable on anyone else’s device.

In other words, did you hear that thunder in the background? Yes, this means that the Tor book you bought on your Kindle will soon be readable on a Nook or iPad or Sony E-Reader or any other reading device.

The implications are huge. (See our Moby post about DRM and Amazon from just two days ago.) It’s a great development for ebook readers, but potentially a devastating one for Amazon, which has been taking huge losses on ebooks and Kindles in a brutal (and hugely successful), years-long campaign to dominate the market and get more and more people trapped by their investment in well-stocked Kindles.

And so you can bet now that at least a couple of the Big Six publishers — and not just their imprints — will follow suit and make a declaration similar to that of Tor/Forge. Which can only mean one thing.

Another Department of Justice lawsuit must not be far behind.