Dan the Man has emerged from his cone of silence, and granted an exclusive interview to Parade Magazine. It's a shame the interviewer covers a lot of old ground previously covered (dude, look forward, not back), but Dan does give an indication of why the new book took so long:

I was already writing The Lost Symbol when I started to realize The Da Vinci Code would be big. The thing that happened to me and must happen to any writer who's had success is that I temporarily became very self-aware. Instead of writing and saying, "This is what the character does," you say, "Wait, millions of people are going to read this." It's sort of like a tennis player who thinks too hard about a stroke--you're temporarily crippled.

It's a 2-page interview, so make sure you click through at the bottom of the first page.