I'm no expert, but...
webOS 2.x Supports mojo. The change to Bing maps also brought the Enyo 1 framework.
To develop mojo, you can use the Palm SDK, which appears to no longer be available on:
...but some of us downloaded it before it vanished.
There is also the PDK, useful for porting esp. iphone apps I think: https://developer.palm.com/content/r..._overview.html
I think the SDK is Windows/Mac based. If you use Linux, there is the Internals WDK:
WebOS Internals PDK - WebOS Internals
Here's the page on general tools: Portal:Tools - WebOS Internals
Enyo 2 is supported by webOS 3.x.x (Touchpad), LG webOS TVs & of course, LuneOS.
To develop for the Pre2, you have a number of mojo options and the Enyo 1 option. As stated above, Enyo 2 may mostly work, but isn't supported, so it depends on the actual code. Also of course, Enyo lets you go cross-platform: Supported Platforms | Enyo JS
EDIT: And yes, I forgot joapp.com, which was very bad of me!
So, technical considerations aside, if you want to develop for the Pre2, there are plenty of options. But if you want to easily make your app available to newer and future platforms, then you are looking at Enyo and maybe Enyo2. I don't know how well webOS 3, LG webOS & LuneOS support Enyo 1 - I suspect pretty well. But if you start with Enyo 2 and ensure the code you use works on the Pre2, then your options for the newer platforms remain open without a major rewrite.
Finally, there is this: ::: LG Developer | Basic HTML App ::: . It's a TV example, but if your app is very simple, then the 'web-app' approach using just HTML is possible across all webOS versions I think.
I'm not a developer, so I'm laying out the available options for those interested in developing for webOS. Your final choice will depend on your deployment plans and the technical considerations of what you actually want the app to do.