another css3 button version. separated out top-bottom and left-right padding values. a width is actually defined in the mixin. and button is centered relative to its parent element.

css3button-centered.scss

@mixin mybutton ($bgdcolor, $fontcolor: #ffffff, $font-size: 12px, $corner: 5px, $padding-tb: 5px , $padding-lr: 10px, $width: 100px) {
  display: inline-block;
	*display: inline;
	color: $fontcolor;
        font-size: $font-size;
	text-decoration: none;
	cursor: pointer;
	vertical-align: middle;
	background-color: $bgdcolor;
	background-image: url('http://d.cyhung.net/images/btn-overlay.png');
	background-image: -moz-linear-gradient(top, lighten($bgdcolor, 10%), $bgdcolor);
	background-image: -webkit-gradient(linear, 0 0, 0 100%, from(lighten($bgdcolor, 10%)), to($bgdcolor));
	background-image: -webkit-linear-gradient(top, lighten($bgdcolor, 10%), $bgdcolor);
	background-image: -o-linear-gradient(top, lighten($bgdcolor, 10%), $bgdcolor);
	background-image: linear-gradient(to bottom, lighten($bgdcolor, 10%), $bgdcolor);
	background-repeat: repeat-x;
	border-radius: $corner;
	box-shadow: 0 1px 2px rgba(0, 0, 0, 0.25);
	text-shadow: 0 -1px 0 rgba(0, 0, 0, 0.25);
	border-bottom: 1px solid rgba(0, 0, 0, 0.25); 
	-webkit-transition: background 0.1s linear;
	-moz-transition: background 0.1s linear;
	transition: background 0.1s linear;
	padding: $padding-tb $padding-lr;
	width: $width;
	position: relative;
	left: 50%;
	margin-left: -($width + $padding-lr)/2;
}	
@mixin mybuttonHover ($bgdcolor:#e6e6e6, $fontcolor:#333333) { 
	background-color: $bgdcolor; 
	color: $fontcolor;
	text-decoration:none;
        background-position: 0 -99px;
}