Try this:
.row > .span3 {
display: inline-block !important;
vertical-align: middle !important;
}
Fiddle: http://jsfiddle.net/EexYE/
You may need to add Diego's float: none !important;
also if span3 is floating and it interferes.
Fiddle: http://jsfiddle.net/D8McR/
In response to Alberto: if you fix the height of the row div, then to continue the vertical center alignment you'll need to set the line-height of the row to be the same as the pixel height of the row (ie. both to 300px in your case). If you'll do that you will notice that the child elements inherit the line-height, which is a problem in this case, so you will then need to set your line height for the span3s to whatever it should actually be (1.5 is the example value in the fiddle, or 1.5 x the font-size, which we did not change when we changed the line-height).