The fun part is that if you find yourself in an inadvertent VFR-into-IMC situation, you can technically log that as IFR PIC even without an instrument rating per 61.51. You may not want to, but you can.
61.51(g) says "
(g)
Logging instrument time.
(1) A person may log instrument time only for that flight time when the person operates the aircraft solely by reference to instruments under actual or simulated instrument flight conditions."
You don't have to be rated, just operating solely by reference to instruments. You have to be rated to file an IFR flight plan, and you have to file an IFR flight plan to legally operate in less than VFR conditions (without an instructor, of course), but there's always the moonless night over the desert or ocean that people talk about, where you're flying by reference to instruments, but still in VFR conditions.