Bootstrap 2.2.2 is out(blog.getbootstrap.com)

  -animation-delay(@delay) { animation-delay: @delay; ...vendor prefixes... }
  .delay-child-animations {
    &:nth-child(2n){ .animation-delay: 50ms;  }
    &:nth-child(3n){ .animation-delay: 100ms;  }
    &:nth-child(4n){ .animation-delay: 150ms;  }
    ... more things here ...
  }

  @mixin delay-child-animations($max-children: 20) {
    @for $i from 1 to $max-children {
      &:nth-child(#{$i}n){ .animation-delay: ($i - 1)*50ms;
    }
  }