Switching between two power supplies with a P channel Mosfet, Is VGS or VDS a problem?
Your circuit is OK.

To understand this, it is best to examine 4 cases: (assume USB ~ 5V, and VIN 0..30 V).

0) VIN low and USB low: everything off, no problem.
1) VIN low, and USB high: The FET will be on with G=0, and S=D=USBVCC. Everything OK, as long as D3 is greater than a 5 V zener
2) VIN high and USB low: This will put VIN (30 V - diode = 29.3 V) on the gate; FET will be off, and D (USBVCC) = 0 is OK; S will be VIN-D3 = 29.3 V. So VGS will be ~0 and OK. No problems.
3) VIN high and USB high: Similar to #2 --FET still off (VGS=0).

There are some potential 'corner' cases -- when VIN is rising and USB is high. You should check that there is no back feeding from VIN to the USB. Your arrangement of D2 &amp; D3 ensures that VGS will remain zero (i.e. FET off), so all is OK. Another case is VIN ~ 4 V and USB high -- in this case, the FET G will be 4-0.7 = 3.3 V, and the FET will be barely on, although power should flow from USB. You should ensure that wen VIN is 'off', it  really is off.