Description of problem:
Desktop with three monitors on two Radeon HD4000 series cards, one 4870 and one 4670.  Two monitors are Samsung SyncMaster 244T and the other is Dell UltraSharp 2405FPW.  All very common monitors, best sellers for their time, very widely distributed.  All are attached by DVI ports and cables.

All three work perfectly as expected in landscape mode (horizontal).  But I have changed to vertical (portrait) mode to conserve room on top of my physical desk.  Now the Dell is working as expected, but both of the Samsungs have subpixel text rendering problems.  This happens no matter whether I attach the monitors to one video card or the other, and it happens regardless of one, two, or three monitors in use.

The symptoms: with any window open containing text -- e.g. a browser or text editor -- subpixel smoothing appears to be getting re-calculated every time the contents of the window change in any way at all.  Change tabs or pages in a browser, or scroll some text, or enter even a single character, and all of the text and other contents on the page can shift and wiggle by subpixel amounts.  Often some characters are left imperfectly rendered and horizontally squashed.  It almost acts like the poor performance of LCDs on an analog connection, but this system is fully digital.

It almost seems as if there is an improper pixel clock setting only when the monitors are in vertical/portrait orientation, or an incorrect internally reported pixel pitch.  Because the display keeps recalculating and redisplaying window contents and the results keep changing, so the entire contents of a browser can appear to "wiggle" when entering text.  If I open my Gmail account and click from one email to another, browsing through my inbox, every time I go to a new email the entire Gmail display shifts a little to one side or the other.

This is terribly visually disturbing and much too similar to the problems with old CRTs or analog LCD displays!

Finally I should add that no matter what subpixel-smoothing settings I use under System Settings > Appearance > Fonts, I still get this problem.  It goes away if I turn off subpixel smoothing completely, but then the text display quality is very poor.

One more final note: just a hint or a hunch that might help developers look in the right places to track this down.  It really seems like somewhere in the driver (or in the monitor information?) there is something that is forcing windows to recalculate their layout upon every single change of content, which seems different from normal.  Either that, or recalculation is the normal state of affairs but normally would not result in visually detectable horizontal shaking/wiggling.  One thing I noticed that might provide an additional clue is that on some websites, Facebook being a notable example, when entering text in any "textarea" box, the browser seems to be getting forced to recalculate the width of that box with every single new character entered.  This causes every new character entered to potentially change which words are wrapped to the next line.  Frequently a given word will bounce constantly from one line to the next and back again, as its word-wrap calculation keeps changing with every new character input.  This is definitely unusual behavior and absolutely annoying to a degree that browsers are nearly unusable for a certain number of applications while this problem is in effect.

Steps to reproduce:
1. Ubuntu 11.04, Gnome Classic Desktop (I can't even get Unity/Compiz to work with this multi-monitor portrait orientation desktop config, it crashes.)
2. AMD fglrx driver version 11.6.  I am about to try 11.7 and will update this report if anything changes.
3. Samsung 244T monitors *must be in portrait orientation to reproduce this problem* on one or more HD4xxx series video cards.  Set all to single-display dekstop, reboot (mandatory according to AMD CCC) then enable Xinerama and reboot once more.

Actual result:
Multi-monitor Xinerama desktop basically works, but on some monitors sub-pixel text rendering keeps changing as window contents change, such that entire window contents can seem to shake and wiggle annoyingly.

Expected result: 
Stable sub-pixel smoothing of text that does not get constantly recomputed or force window contents to shake and wiggle as window contents change.

Attachment: Ugh.  I would have attached atigetsysteminfo.sh output except I just uninstalled the previous fglrx driver and am about to reboot and install the next.  So the .sh script is not present!  I'll be back.