Honesty is definitely good. It worked for Bridget Jones anyway.
Boss: So, why do you want to work in television?
Bridget: I've got to leave my current job because I've shagged my boss.
Boss: Fair enough. Start Monday. We'll see how we go. And, incidentally, at 'Sit Up, Britain', no one ever gets sacked for shaggin' the boss.
In all seriousness, we need more info. Why ARE you applying? Are you looking to expand your skills, try something new, get more money, get away from horrid current colleagues, what? Is it a super-cool company with lots of opportunities or an awesome product line? Tell us the truth and we can help you reword it in a way that sounds good. I worked in corporate HR for four years (two of which were spent as a recruiter) so I can probably help you out.
PM me if you really did shag the boss and don't wanna post that.