I know that tables sometimes cause problems when you're doing SEO (or at least that's what I understand thus far) but does it affect usability? I had planned on using a tabled-layout for the administration area of my content system and I was wondering if that would make things more irritating for the client/admin?

I know that tables sometimes cause problems when you're doing SEO (or at least that's what I understand thus far) but does it affect usability?

Just a myth and nothing wrong with tables in the right places for usability.

I had planned on using a tabled-layout for the administration area of my content system and I was wondering if that would make things more irritating for the client/admin?

I'm using a table based admin layout for the CMS / Catalogue site I'm developing and none of the testers have come up with anything major in the layout.

Once you get the hang of them, divisions can make cleaner markup instead of having a lot of nested tables. However, tables aren't and shouldn't be dead, they just need to be used in the way they were meant to be used: for presenting information. That said, unless your clients are going to be looking at your source code, you can make divs and tables accomplish the same thing, so I don't really think it matters in that regard. Tables are not going to have any effect on your SEO.