Looking through a few documents on Intel's site, "ACPI Uniprocessor" is a default description for the CPU in an ACPI compliant computer.
In several chip-set design guides, Intel refers to the CPU as a Uniprocessor.
Now, as to your specific problem:
When do you get the "CPU not supported" message?
Does the BIOS properly identify the CPU for Front Side Bus and clock speed?
It may be a simple mater of the CPU identifier not being included in the CPU table of the BIOS.
For socket 429 and 478, Intel has managed to increase the complexity of selecting a CPU.
Different motherboards and chip-sets support different Intel CPUs.
At some processor speeds, there are as many as 4 different Intel P4 CPUs. At most, 2 of the 4 will work on a specific motherboard.
The Celeron family has 2 basic models. The standard Celeron CPUs have a 400 MHz FSB.
Celeron "D" CPUs have a 533 MHz FSB.
Older motherboards might not support Celeron "D" CPUs.
Many of the newest Intel based motherboards will NOT support the 400 MHz FSB Celeron CPUs.
If you get the error message when running applications, I would be curious to know what the applications are.
I could make a case that some CAD and other graphics programs would require a Pentium CPU and would not be able to run with a Celeron.
The hardware requirements for the software would detail this.
Chas