Most datasheets show that the power supply rejection ratio (PSRR) of a linear voltage regulator is poor at high frequencies.  This is often a path for conducted immunity issues.  

Do buck switching regulators have better high frequency PSRR?  With the main power supply component being an inductor this makes sense (z=sL, so higher impedance at higher frequencies, thus better blocking).  At least until you hit the parasitic effects of the inductor (parallel capacitance turns it into a tank circuit).  

Just a thought for Friday afternoon...  

Z