There are two symptoms apparently related to an ACPI issue on my computer, as none of these problems happen if I boot with ACPI=OFF (except then the system cannot shut itself off or, in the case of suspend, recovers immediately).
PC is Dell Optiplex GX520 SFF and has an unused onboard dual-monitor integrated video and also an add-in PCI video card (Nvidia Quadro NVS 50 PCI). The monitor is a Hanns-G 1440x900 widescreen LCD connected to the add-in PCI card. I need to run the machine this way because PC has no PCI-Express slot and something is very wrong with the onboard video (cooling related), and the entire computer hangs on GPU faults if set to 1440x900 and a Windowing graphics application (like X windows) uses the onboard video for very long.
If I boot Ubuntu WITHOUT the acpi=off boot option, Plymouth (the Ubuntu boot progress meter splash screen) never shows up. Also, if I try to suspend the PC after logging in, it will turn off but hang when I go to wake it up. Also, the system will just hang if I try to hibernate it, and afterwards will not recover the session if I manually power it down and then back up. This happens even with acpi=ht BTW. Happens regardless of whether I am using the 2D or 3D Unity shell. Only if I use acpi=off however, do I ever see graphical Plymouth (hooray!) and hibernate works except I have to manually power down the system once the System Halted message appears but it actually even recovers when I turn it back on (Suspend also works, although the machine never actually sleeps and immediately recovers to the locked session screen, all due to the consequences of disabling ACPI).
Finding out that ACPI is involved was a breakthrough. This problem happens when running with the Nvidia proprietary driver, with the Nouveau driver, and with the X.Org NV driver under 11.10 Oneiric (didn't happen in 10.10 Maverick, however). Prior to my upgrade from 10.10 (Maverick) to 11.04,11.10 (Oneiric), the X.Org NV driver actually did suspend/recover the system just great, though for that alternate driver to run, I needed to remove the nouveau.modest=0 (causing a non-graphical Plymouth to display). In 11.10, the NV video driver (xserver-xorg-video-nv package) has been removed from the distro by Debian for lack of support from its authors, though it still can be manually installed with dpkg/aptitude and does actually still work otherwise just about fine. I'm so upset it doesn't help anymore!
The fact that a simple app. like Plymouth is affected, which runs very early in the boot process, is encouraging. What other clues, advice, or other system information do you think might help crack this case?