tl; dr: There are two options: Dashcode or Cappucino, where the GUI solution seems to create your user interface in Xcode (Interface Builder) and export it to Cappucino. This solution seems to be supported by the core Cappucino team. The atlas and conservatory are dead.
The rest of this answer consists of quotation marks and links supporting the above conclusion.
I have the same question and did some research.
There are three possible frameworks in which such a tool may be possible. Dashcode, Greenhouse and Atlas. The atlas is dead; the greenhouse has never been more than an experiment.
The dash code is saved. This is a really good tool, and the only platform I found allows you to simultaneously create mobile applications, as well as web applications, from the same code base. (Many mobile app developers, such as titanium, phone screensaver, etc., seem to focus only on mobile devices, not on the desktop and mobile device.)
Dashcode is updated every time Apple releases a new version of Xcode. In addition, the apple does continuous work in this area - for example, iAds Producer really is a super shop. In addition, iBooks Author is also very similar to dashcode. Both of these are specific tools for certain limited target products, but both have javascript frameworks and at least some GUI support.
The producer is really smooth, although he is focused on creating "flash" like ads with javascript.
Dashcode has not been substantially updated for quite some time, so it may be on the way out, but it is still a viable tool.
The boys from 280 North, it seems, are no longer working on a motorola (now a strip lane voucher is currently working), and all work at Atlas has stopped.
The "officially recommended" solution is similar to XcodeCapp. Which, as far as I can tell, is now built into Cappuciono. "Best of all, the new XcodeCapp application automatically creates an Xcode project from your Cappuccino project and makes it easy to place user interface components." (more than a year ago, when they could have mentioned Atlas.)
Here are some links to it, including a recent statement that has been fixed for Xcode 4.4 (which was released this summer, with 4.5 being the current version.)
https://groups.google.com/forum/#!topic/objectivej/mRMiLTP2Yh8
In addition, this following link refers to a site with some great tutorials, and they suggest that you use Xcode: http://www.cappuccino-project.org/learn/tutorials/advanced-hello-world-2/
Here's an older blog post about the situation: http://cappuccino.org/discuss/2011/11/16/cappuccino-0-9-5/
Which contains it in the comments: "Only people who own the Atlas project can make any statements about it, and, unfortunately, we are just not the same people. We are the main developers of Cappuccino, and we run the open source project Cappuccino. Atlas was a commercial product created by a company that no longer exists.
However, do not worry about this. Builder Interface Integration is an awesome and very complete solution. And besides, you can write applications perfectly, without any graphic editor. Or you can even write your own editor in Cappuccino. You have a huge number of opportunities and opportunities at your disposal, so go out and create a cool application. "
And also: “The new way to convert xibs to cibs automatically is the brilliant new XcodeCapp application, which you will find in /usr/local/narwhal/packages/cappuccino/support/XcodeCapp.app . It either struck us as an intuitive and easy way to search or we didn’t have time to make it easier .;) "
This is also a summary of Antoine Mercadal:
"Atlas is a closed-source software that was originally created by the 280N guys and you may or may not know the story, but the fact is that Atlas is more likely dead. We, the Cappuccino community, cannot do anything, unfortunately. The Atlas complaint here is similar to the complaints on the W3C website about IE: it makes no sense.
I personally paid for the Atlas beta and I developed all of my interfaces (about 30 large cibs). I was also very disappointed with Atlas's poor health. This is the price you have to pay for using proprietary beta software: shit happens. I spent a week rebuilding my entire interface under IB. It was a pain, and I drank a lot of coffee this week, but it's really worth it. The result is amazing.
Support for Cappuccino by IB (with XcodeCapp) has now never been a way outside the Atlas. And now the future is guaranteed because XcodeCapp is part of Cappuccino, nib2cib is part of Cappuccino, both of which are Open Source, and no one can ever “close them”.
Atlas was a young product, Cappuccino was also a young product, and like all community-based projects, all participants / tools / members need to be “calibrated” and need to experiment with a few things. I think this is done now, and so we are approaching 1.0.
This release is awesome, there are many fixes and new features, that’s what we should talk about in this thread :) "