As text-align doesn't work I use a simple work around:
.mydiv{
width: 100%;
left: 50%;
}
It's not perfect - but very simple and easy to understand - and you can modify it to suit.
Seems to work ok in IE 6 & FF 2.0.
Here's another method (http://www.andybudd.com/archives/2004/02/css_crib_sheet_3_centering_a_div/index.php) - but I found it not obvious/simple enough (expecially for others who may have to maintain the css down the track - KISS)
HTH
Tim