i have seen around some xorg logs and have seen that the driver doesn't read modelines. then a gentoo user has found out that modifying 3 things in his kernel config has made the driver read once again the modelines. i've diffed the its settings and seen that on my config (modeline scan works for me fine) they were present. the settings are the following:

CONFIG_PCI_MSI=y
CONFIG_ACPI_PROCFS=y
CONFIG_ACPI_PROC_EVENT=y

i suggest to the users that are having problems with the resolution to try out this settings and also to start the atieventsd service before starting xorg and see what happens.