Hang out on forums, #javascript, etc some time and see how many people have problems because their understanding of the DOM begins and ends with the jQuery API. I see it all the time. I've seen big-name sites bug out or slow to a crawl because some dev wasn't thinking about what was going on behind their $()'s.
The DOM API isn't very large/complicated, and it shouldn't be hard to MOSTLY memorize it. It isn't a "gotcha" question though, a good interviewer is not gonna ding the heck out of you if your forget bits of it (most web devs do spend most of their time with jQuery, YUI, etc after all) - it's about whether you understand what's going on behind the scenes.
The DOM API isn't very large/complicated, and it shouldn't be hard to MOSTLY memorize it. It isn't a "gotcha" question though, a good interviewer is not gonna ding the heck out of you if your forget bits of it (most web devs do spend most of their time with jQuery, YUI, etc after all) - it's about whether you understand what's going on behind the scenes.