Forwarding this bug from Ubuntu reporter Mathieu Trudel:
http://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-ati/+bug/541501

[Problem]
External display shows massive horizontal offsetting (shivering of some sort) between lines. See http://www.youtube.com/watch?v=8Ky9Yk3pNuI

This problem began a few weeks ago [about when ubuntu switched from .32 to .33 drm - ed]

[Original Report]
As described, the external display shows massive horizontal offsetting (shivering of some sort) between lines. See http://www.youtube.com/watch?v=8Ky9Yk3pNuI

The effect on actual legibility of the screen depends on resolution and refresh rate. Tested on two independent T60s by swapping the hard drive.

Someone else reported the same effect on a M6 LY (lp #539262)

Note that this behavior was new from at most a couple of weeks ago, at the beginning of March this system was running fine with modesetting enabled and no flickering.

Booting with radeon.new_pll=0 had no effect on the behavior, it flickered just as much

In the interest of full disclosure, I may only have this hardware available to me until the end of the week (Fri Mar 26th), so I might not be able to offer more testing past that point.

'radeon.modeset=0' really did cause the problem to completely disappear. I left my system on overnight and the flickering seems to have return, even while radeon.modeset is set to 0. 

'radeon.dynclks=0' alone doesn't have any more effect than 'radeon.new_pll' did. I tried it again in addition to disabling modesetting, but did have the flickering reoccur after about an hour.

Architecture: i386
Date: Thu Mar 18 11:53:22 2010
DistroRelease: Ubuntu 10.04
EcryptfsInUse: Yes
InstallationMedia: Ubuntu 9.10 "Karmic Koala" - Release i386 (20091028.5)
MachineType: LENOVO 2623D6U
Package: xorg 1:7.5+3ubuntu1
PccardctlIdent:
 Socket 0:
   no product info available
PccardctlStatus:
 Socket 0:
   no card
ProcCmdLine: BOOT_IMAGE=/boot/vmlinuz-2.6.32-16-generic root=UUID=8a9bc647-d729-45f0-bc68-51b0b137e213 ro quiet splash
ProcEnviron:
 PATH=(custom, user)
 LANG=fr_CA.utf8
 ProcVersionSignature: Ubuntu 2.6.32-16.25-generic
SourcePackage: xorg
Symptom: display
Uname: Linux 2.6.32-16-generic i686
dmi.bios.date: 09/12/2008
dmi.bios.vendor: LENOVO
dmi.bios.version: 79ETE3WW (2.23 )
dmi.board.name: 2623D6U
dmi.board.vendor: LENOVO
dmi.board.version: Not Available
dmi.chassis.asset.tag: No Asset Information
dmi.chassis.type: 10
dmi.chassis.vendor: LENOVO
dmi.chassis.version: Not Available
dmi.modalias: dmi:bvnLENOVO:bvr79ETE3WW(2.23):bd09/12/2008:svnLENOVO:pn2623D6U:pvrThinkPadT60:rvnLENOVO:rn2623D6U:rvrNotAvailable:cvnLENOVO:ct10:cvrNotAvailable:
dmi.product.name: 2623D6U
dmi.product.version: ThinkPad T60
dmi.sys.vendor: LENOVO
system:
  codename:           lucid
 architecture:       i686
 kernel:             2.6.32-16-generic

01:00.0 VGA compatible controller [0300]: ATI Technologies Inc M52 [Mobility Radeon X1300] [1002:7149]