I had a longer post here, but Blogger decided to erase it so I'm just going to post the link.
WotC announced today that Modern will be a supported Eternal format ("eternal" meaning no sets rotate out, similar to Legacy or Vintage). You may remember Modern was one of the Constructed formats for the Magic Online Community Cup.
Modern basically allows cards from Core Sets and expansions starting with 8th Edition and Mirrodin through the present. The oldest cutoff is 8th Edition for Core Sets and Mirrodin for expansions.
Modern also has a ban list associated with it.
You can find the ban list and announcement and reasonings for the changes here: http://www.wizards.com/Magic/Magazine/Article.aspx?x=mtg/daily/ld/155
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