<p>I need to create a nice installer for a Mac application. I want it to be a disk image (DMG), with a predefined size, layout and background image.</p>

<p>I need to do this programmatically in a script, to be integrated in an existing build system (more of a pack system really, since it only create installers. The builds are done separately). </p>

<p>I already have the DMG creation done using "hdiutil", what I haven't found out yet is how to make an icon layout and specify a background bitmap.</p>

Don't go there. As a long term Mac developer, I can assure you, no solution is really working well. I tried so many solutions, but they are all not too good. I think the problem is that Apple does not really document the meta data format for the necessary data.
Here's how I'm doing it for a long time, very successfully:
Create a new DMG, writeable(!), big enough to hold the expected binary and extra files like readme (sparse might work).
Mount the DMG and give it a layout manually in Finder or with whatever tools suits you for doing that. The background image is usually an image we put into a hidden folder (".something") on the DMG. Put a copy of your app there (any version, even outdated one will do). Copy other files (aliases, readme, etc.) you want there, again, outdated versions will do just fine. Make sure icons have the right sizes and positions (IOW, layout the DMG the way you want it to be).
Unmount the DMG again, all settings should be stored by now.
Write a create DMG script, that works as follows:
It copies the DMG, so the original one is never touched again.
It mounts the copy.
It replaces all files with the most up to date ones (e.g. latest app after build). You can simply use
mv
or
ditto
for that on command line. Note, when you replace a file like that, the icon will stay the same, the position will stay the same, everything but the file (or directory) content stays the same (at least with ditto, which we usually use for that task). You can of course also replace the background image with another one (just make sure it has the same dimensions).
After replacing the files, make the script unmount the DMG copy again.
Finally call hdiutil to convert the writable, to a compressed (and such not writable) DMG.
This method may not sound optimal, but trust me, it works really well in practice. You can put the original DMG (DMG template) even under version control (e.g. SVN), so if you ever accidentally change/destroy it, you can just go back to a revision where it was still okay. You can add the DMG template to your Xcode project, together with all other files that belong onto the DMG (readme, URL file, background image), all under version control and then create a target (e.g. external target named "Create DMG") and there run the DMG script of above and add your old main target as dependent target. You can access files in the Xcode tree using ${SRCROOT} in the script (is always the source root of your product) and you can access build products by using ${BUILT_PRODUCTS_DIR} (is always the directory where Xcode creates the build results).
Result: Actually Xcode can produce the DMG at the end of the build. A DMG that is ready to release. Not only you can create a release DMG pretty easy that way, you can actually do so in an automated process (on a headless server if you like), using xcodebuild from command line (automated nightly builds for example).