I just noticed that the legal monitor resolutions in the kickstart xconfig command do not include (e.g.) 1280x800. This is WXGA resolution, and it is becoming very widely used on real machines. At a minimum, it would be good to add this resolution to the legal list. Better still, add all of the resolutions that are legal at the corresponding firstboot screen. These should be added to anaconda and also the system-config-kickstart utility. There may be a good reason not to allow more flexible resolutions. I cannot think of one, but perhaps there is something I don't see here. My sense is that anyonw building a kickstart file is knowledgable enough to understand that they need to pick a resolution that is appropriate for the target device(s). Even if the more general list isn't added, adding WXGA is probably worth doing.
I believe this should be fixed now. Please check FC6 test 2 and let me know if it's still not allowed. There's been a lot of X work recently.