I just tried to install OpenSUSE 11.3 on sda4 which is approximately the 2nd half of a 1TB drive, with a pre-existing Win7 installation on the 1st half. The installation summary (before it made any changes) told me "The bootloader is installed on a partition that does not lie entirely below 128GB. The system might not boot." Naturally, not keen on having a computer that won't boot, I aborted the installation.

Of course it's not below 128GB -- Windows is there.

Is this a serious problem? Is there a workaround that is straightforward enough for someone who has very limited experience with GNU/Linux and isn't a comp-sci student?

Yes, this is a serious problem. The Linux kernel only supports up to 1024 cylinders of data on the /boot partition. If the partition is bigger than 1024 cylinders, the computer definitely won't boot.

So basically I'm S.O.L.

Since you are more familiar with windows, you might go to the neosmart site and download EasyBCD to boot Ubuntu. I haven't tried it on a disk that size but it might work:

http://neosmart.net/dl.php?id=1

Rubbish.
The boot loaders used to use a x86 call that had this limitation - because of BIOS limitations. They now check if the extended calls are supported by the BIOS and use that if so. All BIOSs shipped in the last several years support the extended functions.
That warning message sounds like it's to alert you if you have a BIOS that might not handle big disks. Obviously the OP can see the disk, so I'd just ignore the message (it does say "The system might not boot").

Boot loaders are easy to recover anyway, including Win7.

That's encouraging. It is a new (Dell) system. I've had it since December 2009. I'm guessing since it came with the 1Tb drive, the BIOS must be designed to handle disks that big.

I'll go through openSUSE's website with a fine-toothed comb to see if I can find any information about the warning.

The only thing I found was this: Installing on LARGE disks. But it might as well be written in Mandarin for all I understand it. I'm not even sure it's relevant to my issue.

The link you posted refers to arrays of disks over 2TB. Do you have more than one disk? Have you tried installing Opensuse again? You could load the cd and boot from it to a live session and post your partition information here. Open a terminal and login as root, usually by typing "su" or "su -" (without quotes) or you may be able to login as root by simply typing "root" at the prompt. Don't know if a password is used on Opensuse LiveCD, probably "root" if any is asked. Could try just hitting the enter key. To get partition information type: fdisk -l (lower case Letter L) and post the output here.

You should see output with filesystem type, any partition with hpfs,ntfs will be windows so don't install Opensuse to those partitions. You will be asked where you want to install and for other info. You will also be asked about bootloader installation. If you are unsure during any step, it's best to stop. I would also suggest you take notes of steps so that you have some details to post in case of problems.

I do have a 2nd 1Tb drive, which is a recent addition and intended to hold data. I suppose I would be open to installing Linux on that if that would be simpler (or even possible).

I haven't tried installing again, but I will boot from the live-CD again soon and post the partition information as you described.

syg00 said what I was thinking. If your systems is old then you may have an issue with the 127g limit.

Quick update on this: I put openSUSE 11.3 on the 2nd hard drive, left win7 on the 1st, dual-boot works perfectly and I didn't even have to edit the menu.lst.

Thanks, everyone, for your input!