On Engadget, Richard Lawler paraphrases a NYT theory on how the companies identified as participating in the NSA's PRISM program are able to deny participation without technically lying; this is followed up with a quote from Google's chief legal officer denying this theory:
So why the quick denials about something the companies listed (including AOL, parent company of Engadget) may actually have ties to? Because FISA requests are by their nature secret, the report claims employees that deal with the requests can't discuss the details, even with their fellow employees. Notably, although companies must by law respond to the requests, they're not legally obligated to make it easy, and the article points out Twitter as a company that has declined to participate. Because of that, even if PRISM is more a streamlining of bureaucratic processes than a government backdoor into your Candy Crush Saga level, the semantic differences of company denials may not sit well with users, much less citizens voting for the officials who oversee the programs.
Update: Google Chief Legal Officer David Drummond has chimed in once again via a post on Google+, denying (again) that the government has any access to Google servers. That includes directly, through a back door, or any kind of "drop box" as the Times report mentions had been discussed. Meanwhile, CNET has an alternate source who corroborates the company's claims of no direct access, describing the system as a "formalized legal process."
NYT explains how tech companies allow PRISM, yet deny 'direct server access' happened (update)