Hi Avatar,
I don't know which is the target device here, but I just quoted an example of cyclone device to indicate PLL Input/Output frequency limitation, which I faced with cyclone device. I just wanted to indicate that target device will also affect the design if PLL is to be used. I also did a similar test for megawizard (just like you did) and apparently Altera is improving on the PLL limitations. With new devices like Cyclone III/Stratix III, you can get as low frequencies as 1.5 KHz (0.0015 MHz)! (Not verified in the Hardware though, but it should work as Megawizard is not restricting this setting).
This is also new for me and worth taking note of. :)
Thanks
BD