Sounds myth-tacular to me! - I've also heard that neons around the number plate can stop the lazer working - which I've since had confirmed as complete bo11ocks!
You can buy plates that reflect the flash though - illegal they are, but pretty easily available on-line, or there's a spray you can get on e-bay that you spray on your plates - they both work - I've seen the proof 1st hand when we did a little experiment. Only thing is you're easily caught 'cos if you're driving with the sun low behind you it reflects the light and blinds the poor bugger behind you - and if that poor bugger happens to be a John Q Law you're nicked sunshine!