It’s still a work in progress, so although you can:

• open legacy projects such as Apple’s Sample Code (Mac version works best)
• execute NewtonScript
• build packages
• tether your Newt to download packages and debug

you can not:

• use resources in the resource fork of a file as freely as classic mac OS
• lay out a view template with a graphical editor
• create plain C functions
• expect it to be bug-free!

If you want to get involved with development, head over to Github