Helena115: I'm curious how this worked out for you, because I was thinking about doing the same thing, trying Joomla in a subdomain that is. Especially since I've always hand coded HTML sites, I'm both new and very leery about PHP programming, especially after trying wordpress and finding a simple thing like using GIF was causing the image transparencies and pallet to get mangled, due to a flaw in something called the PHP GD library. I did find a workaround, but the experience proved to me it would be wise to develop in a private area of my hosting service, so any problems with PHP could be resolved, and I'd have less chance of surprises when I migrate the site from my subdomain back to the root. So did this work out for you?
I've also been concerned that wordpress interface for adding media to pages, apparently following some new guidelines which I think are plain STUPID, uses full paths to images and graphics instead of relative paths. So I had to get around that too, but you can see where that practice would just make migration more of a pain. Did you find Joomla gave you a similar headache?
As you might guess from this question, I'm here because having tried WORDPRESS as a time saving alternative to replace my HTML site, I've come away feeling its wasted more of my time than its saved. So now I'm considering Joomla instead.! :-)