Actually, vertical-align:middle puts the center of an image at 1/4 em above the text baseline, not at the center of the line height. This doesn't match what absmiddle looks like. I have found the following CSS to approximate 'absmiddle' better:

IMG.absmiddle {
vertical-align: middle;
margin-bottom: .25em;
}

You might be tempted to adjust the top margin by -.25em so as not to muck with the line height, but this can cause problems with elements above, such as lines in tables, so I don't do it.