I recently received some mega328PB TQFP samples and populated a slightly modified mega328P test board. The two chips are nearly pin-compatible, the differences are are shown below.
pin mega328P mega328PB --- -------- --------- 3 Gnd PE0 6 Vcc PE1 19 ADC6 PE2/ADC6 22 ADC7 PE3/ADC7
So far, I have not been able to get the mega328PB to work with a 16MHz crystal even though the same board with a mega328P and the same crystal and load capacitors works perfectly. There also appears to be some odd behavior on the mega328PB with respect to the Clock Failure Detection circuitry and the clock prescaler. I set the fuses for CLKOUT and full-swing oscillator with a 16MHz crystal. Oddly, the measured frequency on the CLKOUT pin was the same (1MHz) irrespective of the states of the CFD and Divide-by-8 fuses. Furthermore, the result was the same if I removed the crystal entirely. When put in an 8MHz crystal, it behaved closer to what I expected except that the CFD fuse doesn't seem to do anything - it happily operates without a crystal (at 1MHz) irrespective of CFD.
I opened an issue with Atmel on the subject but I haven't heard back yet.