This is code that runs using Sass 3.2 prerelease and something like this will be in compass soon.

0_usage.scss

@include keyframes(appear-and-roundify) {
  0%   { opacity: 0; @include border-radius(2px);  }
  100% { opacity: 1; @include border-radius(10px); }
}

1_output.css

@-moz-keyframes appear-and-roundify { 0% { opacity: 0; -moz-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -moz-border-radius: 10px; border-radius: 10px; } }

@-webkit-keyframes appear-and-roundify { 0% { opacity: 0; -webkit-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -webkit-border-radius: 10px; border-radius: 10px; } }

@-o-keyframes appear-and-roundify { 0% { opacity: 0; -o-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -o-border-radius: 10px; border-radius: 10px; } }

@-ms-keyframes appear-and-roundify { 0% { opacity: 0; -ms-border-radius: 2px; border-radius: 2px; }
  100% { opacity: 1; -ms-border-radius: 10px; border-radius: 10px; } }

@keyframes appear-and-roundify { 0% { opacity: 0; border-radius: 2px; }
  100% { opacity: 1; border-radius: 10px; } }

2_keyframes.scss

@mixin keyframes($name,
                 $moz: $experimental-support-for-mozilla,
                 $webkit: $experimental-support-for-webkit,
                 $o: $experimental-support-for-opera,
                 $ms: $experimental-support-for-microsoft,
                 $khtml: $experimental-support-for-khtml,
                 $official: true)
{
  @if $moz {
    @include with-only-support-for($moz: true) {
      @-moz-keyframes #{$name} { @content; }
    }
  }
  @if $webkit {
    @include with-only-support-for($webkit: true) {
      @-webkit-keyframes #{$name} { @content; }
    }
  }
  @if $o {
    @include with-only-support-for($o: true) {
      @-o-keyframes #{$name} { @content; }
    }
  }
  @if $ms {
    @include with-only-support-for($ms: true) {
      @-ms-keyframes #{$name} { @content; }
    }
  }
  @if $khtml {
    @include with-only-support-for($khtml: true) {
      @-khtml-keyframes #{$name} { @content; }
    }
  }
  @if $official {
    @include with-only-support-for {
      @keyframes #{$name} { @content; }
    }
  }
}

3_experimental.scss

@mixin set-experimental-support($moz: false, $webkit: false, $ms: false, $o: false, $khtml: false) {
  $experimental-support-for-mozilla: $moz;
  $experimental-support-for-webkit: $webkit;
  $experimental-support-for-microsoft: $ms;
  $experimental-support-for-opera: $o;
  $experimental-support-for-khtml: $khtml;
}

@mixin with-only-support-for($moz: false, $webkit: false, $ms: false, $o: false, $khtml: false) {
  // Capture the current state
  $original-moz:   $experimental-support-for-mozilla;
  $original-webkit:    $experimental-support-for-webkit;
  $original-o:     $experimental-support-for-opera;
  $original-ms: $experimental-support-for-microsoft;
  $original-khtml:     $experimental-support-for-khtml;

  @include set-experimental-support($moz, $webkit, $ms, $o, $khtml);

  @content;

  @include set-experimental-support($original-moz, $original-webkit, $original-ms, $original-o, $original-khtml);
}

Just want to know, when will this mixin become part of compass? As almost a year has passed, still don't find anything about animation from the document.

@neekey Compass' documentation covers the latest stable version (0.12.2) whereas this feature is for 0.13, which is currently in alpha. Animations will be added to Compass and documented when 0.13 becomes the latest stable version. You can keep up-to-date with new versions of Compass here:

http://rubygems.org/gems/compass/versions