Symbian Foundation has created a purely open source Eclipse-based development environment for WRT app creation. More info at the Symbian blog. Download here.
If you’re completely happy with Aptana, there’s might not be a reason to look at the open source solution.
The main goal with this product is to provide an open source base to kick start a community to rapidly innovate web development environments for mobile device app creation. If that sounds interesting to you, you should definitely take a look.