@hansibull: that crystal run? what are you thinking, was that to prove a point?
It was a big design mistake I did before I knew the 328PB lack the following option. However it did
infact work on the 48PB, 88PB and 328PB (all bough from Digikey). This design is no problem if the MCU supports full swing, but in this case it's very risky indeed.
Everywhere the power supply traces run act like an antenna, they tend to be resonant at specific frequencies. There are analogies to bridge building. The Tacoma Narrows Bridge collapse was what they use to show in physics classes. To work properly the bypass capacitor needs to be placed at the ends of the power distribution traces, and scattered along the trace (e.g. at each IC is good). To keep a bridge from resonating we might add weights to the tension cable to shift its resonant frequency (though I'm not a bridge designer).
First of all, I'm not trying to defend bad design practices at all. I'm a big fan of doing things "properly". However the AVR microcontroller is very tolerant to _really_ poor designs. I've seen a commercial product where an ATmega328P in a DIP package was soldered on a circuit board without any bypass caps at all! It's just terrible indeed, but it did in fact work.
When designing less advanced PCBs at work I usually stick with the ATmega324P in TQFP44 package. I use a total of four 100nF caps, one located on each side. I make sure the ground return path is short, and that the external oscillator is placed as close to the MCU as possible.
Last winter we tested some of the boards I've designed at a norwegian certification organ (much like UL and Intertek). In the test chamber they radiated radio waves from 30 MHz to 6 GHz with a peak power of about 150W or so. None of the microcontrollers failed during the test, even though some of the boards were two layer design without metal shielding.
What I'm trying to say is that even though one should take the laws of physics into account when designing a PCB this is just a slow 8-bit micro, not a sub gigahertz 1152 pin FPGA with ten different power rails.