There are a lot of excellent Joomla themes available for download. Is there a 'best method' for converting for SS use or do they just not work?

Well at the end of the day all a theme generates is html so it doesn't matter if its in Wordpress, or SilverStripe or Joomla since all can output HTML to a page.

However the 2 systems have totally different template engines and data structures so it will take some effort to port any themes across. It can be done (I've migrated old Joomla sites to SS) but I only barely used the CSS from the Joomla template, I redid most of the actual template code.

Thanks again for your reply.
I guessed as much. I'll have to get my hands dirty with this one and really get some practice in for template creation. Sadly the official book contains very little about themes and templates.
Would there be a chance in the future of the community producing some 'base themes' that can be adjusted for colour scemes, etc?
Simple ones like left navigation, top navigation, 2 column design, 3 column design.
Web builders could choose a base layout and be creative with just the CSS. Take a look at http://andreasviklund.com/templates/

GRiiDWeb CSS multiple column layouts are pretty simple, you can find a huge resource of them on sites like http://layouts.ironmyers.com/. If you look at the tutorial theme you can see the SilverStripe specific controls you will need to add to your theme then style it. We are looking at for our next iteration of the default theme providing a complete framework (3 col, 2 col, banners) with the theme to allow people to easily customize it but this is a wee way off.

Thanks again Will,
I like the future idea of having a theme with switchable layouts. It's great for client demos to show them how they can have different looks and layouts.