Hey there,

After a week or so (read: after passing 10 minutes a day, for real) of work I managed to layout the most sensitive components of the circuit, that are in order of layout:

• the chip bypass caps (keep them as close as possible to the chip)
• the crystal (keep traces as close as possible to chip and of same length)
• the USB connector and transient suppressor (differential USB signal traces must be laid out as short and as equally long as possible, avoiding signal under them, some says they should also be laid at an angle to avoid some distortion on the signal due to patterns in the fabric of the FR4 ,but we're talking about USB Full Speed here, not USB3, so it's fine if you don't do that)
• the LDO and bulk bypass caps
• the power signals filters (USB shield filter to ground that also should act as "star ground", LC filters with ferrite beads to create clean analog and digital power signals)

I want to keep a 1 inch spacing between the two headers, but placing the USB connector centered on the left side of the board forced me to put the chip a bit off from the center of the board, which in turn led me to "push" the upper breakout header up of 0.1 inch (to leave room for the crystal). I'll fix this by moving the clock a bit to leave just the right space.

The tricky part now is mapping the pins in a convenient way, that is so to have the less distance between chip pin and header pin to ease the layout process; also, I need to take extra care of the analog power signals so to avoid any noise in them.