Found this on the Apple site:
The power-on self-test resides in the ROM of the computer. Only computers that are based on the ROM-in-RAM (New World) technology, such as the iMac, Power Macintosh G3 (Blue and White), Power Mac G4 (PCI Graphics), PowerBook (FireWire) and PowerBook G3 Series (Bronze keyboard) will generate these beeps. This test runs whenever the computer is powered on after being fully shut down (the power-on self-test does not run if the computer is only restarted).
If a fault is detected during the test, you will not hear a normal startup chime. Instead, the system will beep as explained below. If you experience one of these beeps, you should call your Apple-authorized service provider for additional troubleshooting assistance.
1 beep = No RAM installed/detected
2 beeps = Incompatible RAM type installed (for example, EDO)
3 beeps = No RAM banks passed memory testing
4 beeps = Bad checksum for the remainder of the boot ROM
5 beeps = Bad checksum for the ROM boot block
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