via http://rjlipton.wordpress.com/2009/12/26/mathematical-embarrassments/" [Broken]:

i didn't think the invariant subspace problem would be an embarrassment. i guess it's not all that hard for finite dimensions. only one person mentioned goldbach's conjecture, & nobody said anything about perfect numbers. they seem pretty easy to understand but hard to say interesting things about. same with fermat's last theorem & the 4-colour problem