There is a new report online stating that the release date for the IMAX version of Half-Blood Prince has been delayed.  It is now said to come out July 29th.

The film will open in 35mm theaters on the 15th; however, it will not open in IMAX theaters until the 29th. Basically what happened is IMAX Corporation signed a contract with Paramount guaranteeing them a 4 week exclusive window if they filmed parts of Transformers 2 with IMAX cameras onto IMAX film (something that is quite a bit more expensive). Unfortunately that was back when Harry Potter was due to open November 2008 and when Warner Bros. pushed back the opening date Paramount was not willing to renegotiate the contract it had signed with IMAX.

This has not been confirmed by WB yet so stay tuned!

UPDATE: It has been confirmed. Additionally, one IMAX in New York and another in Los Angeles, will play the film from July 14th.