This is totally unconfirmed, but Japanese online retailer Rakuten is listing Metal Gear Online as a standalone title, out next month. According to the Rakuten listing, the game is slated for a July 17th release in Japan and is priced at ¥1,800 (US$16.60). At the Metal Gear 20th Anniversary party last year, Konami hinted at it being a stand alone, but then later decided to include MGO with Metal Gear Solid 4. Guess this is Konami's way of making sure those who don't pick up MGS4 can still enjoy manapult action. We've contacted Kojima Productions and will let you know if we hear anything.
Metal Gear Online Standalone [Rakuten via ２ちゃんねる Thanks, Muu!]