The culprit behind this is not Quantum or SPECTRE or even SMERSH, but, instead, is MGM, which is desperately attempting to avoid complete bankruptcy.
Said the Wilson/Broccoli statement, "Due to the continuing uncertainty surrounding the future of MGM and the failure to close a sale of the studio, we have suspended development on 'Bond 23' indefinitely. We do not know when development will resume and do not have a date for the release of 'Bond 23,'" stated Michael G Wilson and Barbara Broccoli jointly."
Unfortunately the Bond film rights are half-owned by the studio, so it's not like the producers can bring the project elsewhere.
UPDATE: According to Deadline.com, that may not be entirely true. Click over to Earths Mightiest for a possible explanation for Bond 23 being put on hold. Just click on the image below.