<p>Where can I test HTML 5 functionality today - is there any test build of any rendering engines which would allow testing, or is it to early? I'm aware that much of the spec hasn't been finalised, but some has, and it would be good to try it out!</p>

Ones that are built using a recent webkit build, and Presto.
Safari 3.1 for webkit
Opera for Presto.
I'm pretty sure firefox will start supporting html5 partially in 3.1
All support is extremely partial. Check
here for information on what is supported.