When you do an internet search for Ostara or Eostre, a lot of what comes up tells you how Eostre isn't a real Goddess, how she was made up by so and so, or comes from this. A great, and also non-inflammatory, article about this can be found in About.com's Wicca and Paganism section.
Regardless of what you believe, we still celebrate Ostara because of spring, and the new beginnings it brings. On Ostara we celebrate the Goddess and the God in their sacred joining, the union that will produce the Oak King in 9 months time.
Do you believe that Eostre was a real Goddess? Does it matter to you if she was or wasn't?