XCode largely uses standard Unix tools for a lot of its work. iPhone applications are compiled using GCC 4.0, it uses gdb for debugging, so it should be possible to set up Eclipse to at least compile applications.
If you look in the Build section of the Project Info pane (select the project, hit the Info button in XCode, choose the Build tab from the top) you can see many of the options.
I'm not sure what is required to copy a file to the simulator and attach a debugger to do a test run, it might not be worthwhile to jump through all the hoops and Apple may require an XCode generated build to accept an app into the App Store.
It may be the case that for regular code editing you can work reasonably well in Eclipse, then switch to XCode for interactive debugging.
Most of the existing Eclipse plugins seem to be oriented towards developer iPhone-aware web applications, so I'm not sure if you'll get any help there.