I have two Viewsonic 1920x1080 monitors that work.
Go to TigerDirect.com and pull up the Viewsonic models and they list the frequencies. The only ones by Viewsonic that didn't do 50Hz (they were 56Hz) were the 2 older "square" ones. All the widescreen models I checked seemed to be 50Hz, but do check the models yourself.
I'm very happy with the ones I have and they work with PAL screenmodes on my A4000 (with IndivisionAGA, so it would anyway), Minimig and A2000 (using an "Amber" SDFF - which uses the same Amber chip as the A3000).