Anonymous asks: "How long, in your opinion, will it take for Republicans to embrace a David Cameron-style leader?"
I don't have a "when" for you, but I'll give you some "how" answers.
I'd say that there are two things that generally drive party change. One would be major demographic shifts in the composition of the party -- adding Anglos from the South to the Republicans and subtracting them from the Democrats in the 1950s-1990s certainly had series effects on party policies, as did the addition of various urban ethnic groups into the Democratic Party (various European ethnic groups in the 19th and early 20th centuries, then African Americans from the Great Migration) in previous generations. So if you see anything like that coming, that could shift things dramatically.
The other thing would be electoral pressures to moderate in the face of losses. That's not going to happen in a major way anytime soon, given GOP successes in 2010, but it could happen around the margins...well, at some point. If Barack Obama wins reelection, at least some Republicans are going to notice that Democrats will have won the most votes in (then) five of six presidential elections and start thinking about change. But those voices may well lose out to others who will conclude that Romney or Pawlenty or Daniels or whoever was too moderate, and Republicans did better in 2010 when they ran purist campaigns. Remember: Democrats started nominating William Jennings Bryan and getting clobbered in 1896, and were still doing it in 1908.
What else? Should Republicans win in 2012 and actually have to govern, it's certainly likely that they would react by dramatically ramping down the rhetoric, and possibly moderate some policy positions. Oh, and a fourth thing: it's of course possible that individual Republicans or GOP-aligned groups could change their minds about things and change the party from the bottom (or middle) up. It's hard to believe that the GOP isn't going to moderate on sexual orientation issues over the next decade or two, at least not if generational effects on public opinion continue to be as strong as they have been. That happens, too: see the Democrats' positions on those issues from 1990 to now, or the Democrats on feminist issues from 1965-1980.