Joomlashack's Inspirion Joomla Template is one of the most innovative templates currently on the market. Its built using the 960 grid, a CSS system that gives you a tableless layout and a fast loading site, great for SEO!
Its one of the first commercial professional templates to validate XHTML Strict. This is a high benchmark to meet, and provides a great foundation to help you site rank in search engines.
On top of that, Inspirion has a full set of core Joomla template overrides. This means that your Joomla 1.5 site will be shown with no tables at all and makes use of semantic headings (h1-h3). I'll give you one guess why that is good!
I think that the Inspirion template might be the best template around for SEO right now. Making sure your template helps your SEO efforts, rather than hurts them, involves lots of factors, tabless layouts, valid code, semantic headings, SEO-friendly logo replacement- Inspirion has it all.