It is possible to apply for an MBA right out of undergrad, but it is extremely difficult gain admission.
To give you an idea:
At Fuqua 99.5% of the entering class has full-time work experience.
At Darden 100% of the entering class has FT WE.
At Wharton only 2% of the entering class has 0-2 yrs of FT WE.
At Tuck 100% have FT WE (2008 stats)
I have a friend that was accepted to a top 3 program straight out of undergrad, but he had an EXTREMELY competitive profile. He was president of his fraternity, president of a student club, varsity athlete, double major (4.0 in both) at top 10 undergrad, 2 summers of internship work with a top 10 fortune 500 company, high GMAT (all I know is that it was 750+). You get the point.
There is also the Harvard 2+2 program that accepts you on a deferred basis and allows you to work 2 years before joining HBS.
If you have the right profile, the right story, and a very clear plan you have a chance, but I know that most top schools stress the importance of work experience.