Forwarding this bug from Ubuntu reporter Sean Clarke:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/542660

[Problem]
Black screen shown in X after boot splash 

[Original Description]
Lucid beta 1 fails to boot on my Apple iMac 27" (Core i5 model).

I am happily running Karmic (had to download backport Alsa to get sound working), but Lucid beta 1 fails to boot. You initially see the new purple Ubuntu loader, then it goes black - CD continues to whirl and after it sounds like it is loading in the background it resets and restarts.

Suspect problem with video - I can get it to boot if I select option F6 and "selectnomodes" (I think thats what it was called) - it boots fine (I am running it now).

Question: Would this stop me upgrading from Karmic?

If you need any more info, please email.

Architecture: amd64
Date: Sat Mar 20 12:57:24 2010
DistroRelease: Ubuntu 10.04
DkmsStatus: Error: [Errno 2] No such file or directory
GdmLog1: Error: command ['gksu', '-D', 'Apport', '--', 'cat', '/var/log/gdm/:0.log.1'] failed with exit code 1: cat: /var/log/gdm/:0.log.1: No such file or directory
GdmLog2: Error: command ['gksu', '-D', 'Apport', '--', 'cat', '/var/log/gdm/:0.log.2'] failed with exit code 1: cat: /var/log/gdm/:0.log.2: No such file or directory
LiveMediaBuild: Ubuntu 10.04 "Lucid Lynx" - Beta amd64 (20100318)
MachineType: Apple Inc. iMac11,1
Package: xorg 1:7.5+3ubuntu1
ProcCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/hostname.seed boot=casper initrd=/casper/initrd.lz quiet splash --  nomodeset
ProcEnviron:
 LANG=en_US.UTF-8ProcVersionSignature: Ubuntu 2.6.32-16.25-generic
SourcePackage: xorg
Symptom: display
Title: Xorg freeze
Uname: Linux 2.6.32-16-generic x86_64
dmi.bios.date: 10/30/09
dmi.bios.vendor: Apple Inc.
dmi.bios.version: IM111.88Z.0034.B00.0910301727
dmi.board.name: Mac-F2268DAE
dmi.board.vendor: Apple Inc.
dmi.chassis.type: 13
dmi.chassis.vendor: Apple Inc.
dmi.chassis.version: Mac-F2268DAE
dmi.modalias: dmi:bvnAppleInc.:bvrIM111.88Z.0034.B00.0910301727:bd10/30/09:svnAppleInc.:pniMac11,1:pvr1.0:rvnAppleInc.:rnMac-F2268DAE:rvr:cvnAppleInc.:ct13:cvrMac-F2268DAE:
dmi.product.name: iMac11,1
dmi.product.version: 1.0
dmi.sys.vendor: Apple Inc.
glxinfo: Error: [Errno 2] No such file or directory
system: codename:           lucid
 architecture:       x86_64
 kernel:             2.6.32-16-generic

[lspci]
01:00.0 VGA compatible controller [0300]: ATI Technologies Inc M98L [Mobility Radeon HD 4850] [1002:944a]
    	Subsystem: Apple Computer Inc. Device [106b:00b5]