Simple. IE6 and above will happily center your table with "margin: 0 auto;" if only the page renders in "standards" mode. To make this happen you need a valid doctype declaration, such as
or
True, IE5.5 and below will still refuse to center the table but perhaps you can live with that, especially if the page is still functional with the table left aligned. I think by now users of IE5.5 and below are fairly used to some odd looking websites - but you still need to ensure that those visual glitches don't render your site unusable.
Happy coding!
EDIT: Sorry, I should perhaps point out that you do not have to have a "strict" doctype to get IE6 and up into "standards" rendering mode. I realised it might seem that way from the doctype examples I posted above. For example, this doctype declaration will of course work equally: