Akamatsu Ken announced in his web diary that they can now initiate "the project" freely because the number of the pre-orders of the new OVA Mahou Sensei Negima! Mouhitotsu no Sekai was large enough. He said he could not explicitly disclose what "the project" is from now since the sales data turned Akamatsu's personal desire into a real project .

He had been saying that the final chapter of the OVA series will be released as a movie if the sales are successful.

Source: Akamatsu's diary (Aug 11)
Related topic: Second Series of Negima OVA Announced [Confirmed]

Update
The official announcement will be made on September 17th.

Source: Akamatsu's diary

Updated on September 5th
According to Weekly Shounen Magazine, a "full-length" anime of Negima was announced to be premiered in 2011. They used the term "Negima Saga Final", but it's unknown if this is the tentative title of the anime.

Source: Scan of WSM

Updated on September 9th
Akamatsu Ken said they used the term "full-length anime" because they haven't booked theaters yet. The movie will be the final chapter of the Magic World Arc. He also said "I have no comment on why they wrote 'Negima Saga Final' in the magazine."

Source: Akamatsu's diary

Updated on February 16th 2011
According to Natalie, the Negima movie will be premiered this summer. The "Magic World" arc will be concluded in the movie.

Source: Natalie