Remember last year M10 changed how Core Sets rotate out of Standard and Extended. With M11's release, we have the first time we'll see the new Core Set rotation policy in action.
The old policy, when Core Sets released every 2 years, was that a Core Set would be knocked out of Standard on the release of a new Core Set. For example, when 10th Edition released in July 2007, it knocked 9th Edition, which released in July 2005, out of Standard (so a Core Set was legal about as long as a normal block: 2 years).
The new policy, put into place last year when Core Sets went to a once-per-year printing, is that Core Sets will rotate out with the block immediately preceding it, not when a new Core Set releases. For Standard, this means the previous Core Set stays legal for a few months concurrently with the newest Core Set.
When M11 officially releases this Friday (7/16), M10 does not rotate out of Standard and/or Extended. For a few months, we'll have both M10 and M11 legal in Standard.
When Scars of Mirrodin releases October 1st of this year, Shards of Alara block (Shards of Alara, Conflux, and Alara Reborn) will rotate out of Standard as scheduled (Standard only allows the last 2 years' blocks. That was the Shards of Alara block and Zendikar block, but Scars of Mirrodin starts a new block, thus knocking out Shards of Alara block). M10 is tied to Shards of Alara block, so October 1st is when Standard loses M10.
For now, Extended doesn't toss out M10, even when Scars of Mirrodin releases. It should rotate the same way, though: In 2 years, when Shards of Alara block rotates out of Extended, M10 should go with it.
Questions? Comments? Feel free to post them here!
Until then, enjoy your week. :)