Interesting question! While there are plenty of guides on horizontally and vertically centering a div, an authoritative treatment of the subject where the centered div is of an unpredetermined width is conspicuously absent.
Let's apply some basic constraints:
table-cell, which is of questionable support statusGiven this, my entry into the fray is the use of the inline-block display property to horizontally center the span within an absolutely positioned div of predetermined height, vertically centered within the parent container in the traditional top: 50%; margin-top: -123px fashion.
Markup: div > div > span
CSS:
body > div { position: relative; height: XYZ; width: XYZ; }
div > div {
position: absolute;
top: 50%;
height: 30px;
margin-top: -15px;
text-align: center;}
div > span { display: inline-block; }
Source: http://jsfiddle.net/38EFb/
An alternate solution that doesn't require extraneous markups but that very likely produces more problems than it solves is to use the line-height property. Don't do this. But it is included here as an academic note: http://jsfiddle.net/gucwW/