Guideline 5. Create tables that transform gracefully
Section 508, (Technical Standards) 1194.22
http://www.w3.org/TR/WCAG10/#gl-table-markup
"Ensure that tables have necessary markup to be transformed by accessible browsers and other user agents."
5.1 For data tables, identify row and column headers.
5.2 For data tables that have two or more logical levels of row or column headers, use markup to associate data cells and header cells.
5.3 Do not use tables for layout unless the table makes sense when linearized. Otherwise, if the table does not make sense, provide an alternative equivalent (which may be a linearized version).
5.4 If a table is used for layout, do not use any structural markup for the purpose of visual formatting.
5.5 Provide summaries for tables.
5.6 Provide abbreviations for header labels.
The most important thing to remember when reading through this guideline, is that tables have a purpose. A table should not be used to design the look of your website. Yes, it may make it a lot easier to have everything line up and look uniform across your website, but you will run into problems if you do this. Screen readers and other accessibility devices know how to read tables, or in other words, will read a table in the way that it should be read. If you use tables for the layout of your site, those with disabilities will have trouble deciphering the content of your website.
Tables are to be used for data. When using tables for data, make sure that you identify row and column headers. This will make it readable by screen readers and other devices. In HTML, use THEAD, TFOOT, and TBODY to group rows, COL and COLGROUP to group columns, and the "axis", "scope", and "headers" attributes, to describe more complex relationships among data.
When you use TH to identify headers, you might also want to include an abbreviation by using the ABBR element. This will allow you to fit more information into a smaller cell, but will still allow you to fully explain the content. e.g. (SS# -> ABBR="Social Security Number"
In conclusion, just remember what tables should be used for. It could end up costing your client a lawsuit if you fail to create thier website with these important guidelines in mind.