1. All three models had their speed increased by 83MHz (1.25GHz became 1.33GHz, and 1.42GHz became 1.50GHz).
2. All three models come with 5400rpm hard disks. Previous Mac Minis had 4200rpm laptop hard disks.
3. The Bluetooth module supports Bluetooth 2.0 and EDR (enhanced data rate).
4. The Radeon 9200 video card in the 1.5GHz models has had its video memory doubled to 64 megabytes.
5. The Superdrive (DVD writer) is a newer model that can burn DVD+R dual layer disks.
Apple did not announce this upgrade, because they didn't want to harm sales of the forthcoming Intel Mac Mini. (You'll notice that even now, Apple's web site compares the performance of the Intel Mac Mini with that of a G4 one running at 1.42GHz, not 1.50GHz.)
Because of this, buyers at the time didn't know in advance whether they would be getting an upgraded Mac Mini, or discontinued stock of the older (1.25 and 1.42GHz) versions. However, just about everyone did receive the upgraded computers. If the box says that the Mac Mini comes with iLife '06 rather than '05, it's certain to be the upgraded version.
It may be worth asking sellers to have a look at the Apple System Profiler (select About This Mac in the Apple menu, and click on More Info), which will inform you of the CPU speed and other details. But please don't be offended if a seller refuses to open a brand new, sealed box just to do that.