I forgot to mention. If you hold the option key, when you boot the 1.5 GHz mini, at least with my 22" Dell connected to DVI, it displays everything correctly - 1920x1080. Cursor and icon sizes as they should be.
I've spent a good part of my afternoon patching things and modifying ROMs. All I can say at the moment is that it's the ATI stuff that messes things up.
Yes, not just the "alt-boot" Boot Picker menu, but even Open Firmware looks fine in 1920x1600@60Hz, and even the Happy Mac logo when booting OS 9. The problem starts however right after the Happy Mac logo is done displaying. Of course, OS X 10.2 ~ 10.5 (and probably 10.6 alpha) all show fine, too.
It'd be awesome if that issue was solved with Mac OS 9 for the 1.5GHz mini.
Also, as per request, I'm reposting my experiences here, as well, so that information on this specific issue is not "lost" in the huge Mac mini G4 megathread:
One part of me is still not 100% sure if the following is an overall "revised" / "silent upgrade" mini issue, OR a specific 1.5GHz "Super mini" issue. In other words, I don't know for sure if the 1.33GHz mini is also affected or not. I highly suspect it is, BUT... Only someone with a 1.33GHz mini can tell us for sure.
In my case, I have a DELL U2412M, it's a 16:10 aspect ratio, 24", 1920x1200 @60Hz monitor. Used with the 1.5GHz Mac mini G4 (AKA "Super mini") model.
After using
these ATI Extensions, which are different from what's on both v8 and v9, I got it to recognize a lot of resolutions and frequencies, via DVI. Even 1920x1200, but only @ a frequency higher than 60Hz. But my monitor is limited to 60Hz maximum. I get a black screen if I try such higher frequencies. However, at 60Hz, I do have the option to use 1600x1200 still, which is 4:3, and it just adds some black borders to the side, but otherwise it's a pixel-native resolution that works well, as long as I configure my monitor to display everything with a 4:3 aspect ratio.
Some full-screen games automatically try to switch it to 1920x1200, though, which my Mac OS 9 can only recognize at a frequency higher than 60Hz, and so I get a black screen with them, despite what I set the resolution to. So even my manual fix is not perfect.
Using an old mid '90s CRT monitor I have, though, those ATI Extensions I mentioned lead me to get no picture.
Switching to v9's unmodified ATI drivers instead, then it "works", but only at 800 x 600 resolution, and only at 256 Colors. Some games crash when I play them like this.
Now, when I switch to v8's unmodified drivers, then it works perfectly for (almost) every resolution option it gives me, which is a lot of them! Then the same games that crashed before don't crash anymore, and I can use Millions of Colors.
I hope this helps us keep track of this issue. As you can see, short of using a 1.25GHz and 1.42GHz mini, graphics and monitors can be messy and complicated. Using precisely these 2 mini models, the issue just disappears.