Mr Davis, who will return to the Commons as a backbencher, said he did not plan to become a "single-issue campaigner" on civil liberties.
Asked if the Haltemprice and Howden MP will be given another shadow cabinet role, Mr Cameron said: "He can contribute in the future... I think he's made his point in the way that he wanted to.
"What matters is what's right and standing up and saying what's right as the Conservative Party, throughout this whole argument, has done."
Mr Cameron added: "I will obviously talk to him about what the future holds, but I've got a very strong shadow cabinet. David is a very strong Conservative figure.
"I'm sure there will be many ways he can contribute in the future."
Mr Davis, who was beaten by Mr Cameron in the contest to become party leader in 2005, said: "We have fired a shot across the bows of Gordon Brown's arrogant, arbitrary and authoritarian government."
He said he would return to Westminster on Monday with a mandate "to fight Gordon Brown's vision of Big Brother Britain tooth and nail, to stop 42 days in its tracks, to prevent the disaster of ID cards before it happens, to protect our personal privacy from being ransacked by the ever-intrusive state".
Mr Davis is expected to discuss his future role, if any, with Mr Cameron after he returns but he admitted it was unlikely the Tory leader would invite him back onto the party's front bench.
"I took on board that I would lose my shadow cabinet post and probably my shadow cabinet future," he said. "I accept that."
Mr Davis promised to "put a lot of effort" into opposing 42 days' detention on his return to Parliament.
He told BBC Radio 4's Today programme: "The trouble with this is, from the beginning, the Westminster village hasn't really understood that someone wants to take a stand on a matter of principle that may have some effect on themselves."
Mr Davis also accused the government of "spectacular cowardice" for not fielding a candidate.
But Mr McNulty accused Mr Davis of "vanity" for prompting a by-election, saying: "It doesn't need David Davis to give the country his permission to have a debate on the issue (of 42 days' detention)."
He added that Mr Davis should have remained an MP to "make the arguments" in Parliament.
In light of Gordon Brown's recent admission that it was "absolutely correct" to compare himself to Heathcliff, the romantic hero of the novel Wuthering Heights, Mr McNulty was asked who Mr Davis reminded him of.
He replied: "David Davis? Probably Homer Simpson."
Liberal Democrat leader Nick Clegg said: "The Conservatives are a long way from being defenders of liberty.
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.