The problem, of course, is that at least one recent poll showed that a majority of Republicans have doubts about the president's citizenship, which explains the rise of "post-birtherism." Republican leaders don't want to anger a large section of their base by flatly calling this stuff what it is, which is nuts. So the new approach is to joke about it or to carefully avoid denouncing the idea completely.
The White House didn't force GOP Presidential hopeful Tim Pawlenty to make a birther joke at CPAC. It's also worth noting that three Republican congressional leaders recently gave the same exact answer to questions about the President's faith and citizenship:
Senate Minority Leader Mitch McConnell on Meet The Press last August: "The president says he's a Christian. I take him at his word. I don't think that's in dispute."
http://voices.washingtonpost.com/plum-line/2011/02/birtherism_worst_trap_ever.html