OK, I tweaked the CSS again (using Firefox Developer Tools) and now the vertical alignment of the field labels is displaying correctly in Chrome 27, Firefox 21 and IE 9.
Here is the current rule, at line 122 in the (modified) CSS file wp-members-2012.css:
#wpmem_reg label.textarea , #wpmem_reg label.select,
#wpmem_login label {
left:4px;
top: 4px;
color:#555;
width:210px;
margin-top:4px;
margin-bottom:4px;
padding-top:4px;
padding-bottom:4px;
padding-left:4px;
float:left;
display: block;
font-family: inherit;
font-size: inherit;
line-height: 2.2;
height: 42px;
display: block;
}
This makes the field labels vertically align correctly - at least when using the themes RTTheme17 and VibeCom.
Your mileage may vary!
PS - It would also be nice if the text which says:
*Required field
were slightly lower on the page.
However, this text is without a tag in the current HTML file, so it is probably not possible to change its position by modifying the associated CSS file.