If all the page locations and content are staying the same, probably the only effect you will see is positive - your crawl rate may pick up briefly (there is a theory that if G sees a sitewide change it will increase its crawl rate temporarily to cache the updated versions. I have seen this happen on my own site after changing the look in one section of the site) and the code:content ratio is also important - the less code you have on the page, the better the search engines are supposed to be able to index your content, and switching from tables to tableless CSS should give you this benefit. You may see a short dip as the search engines digest the changes, but that is likely to be very brief. You may also see your
PR tank, but that is more an effect of the way the toolbar is updated and it should not affect your rankings.