Last year AMD officially became an ARM licensee, although the deal wasn't publicized at the time. Fast forward to June 2012 and we saw the first fruits of that deal: AMD announced it would integrate ARM's Cortex A5 core into its 2013 APUs to enable TrustZone support.
Today comes a much bigger announcement: AMD will be building Opteron processors based on a 64-bit ARM architecture. There are no product announcements today, but the 64-bit ARM Opterons will go into production in 2014. Today's announcement is about a processor license, not an ARM architecture license - in other words, AMD will integrate an ARM designed 64-bit core for this new Opteron.
The only other detail we know is that these ARM based Opterons will embed SeaMicro's Freedom Fabric, presumably on-die.
AMD offering ARM based Opterons is really to target the microserver market. As for why AMD isn't using Jaguar for these parts, it's likely that by going with ARM it can lower the development time and cost to get into this market. The danger here is the total microserver market is expected to be around 10% of the overall server market, but that includes x86 + ARM. With x86 as the default incumbent, it's going to be an uphill battle for AMD/ARM to carve out a significant portion of that market.