In the process of "perfecting" my Berliner Weisse, I've tried several different dry ale strains, but the one I like the best is actually a dry lager yeast: Saflager S-23. I don't actually lager the weisse, but I do let it ferment on the chilly side to start (52F for 3 days, raise to 55F for 7 days, then raise to 68F until I'm satisfied fermentation is complete). The yeast character is amazingly clean, so it really lets the tart fruit and sourdough wheat flavors shine through. Sorry I don't have an actual pH measurement of the wort (will have to remember to measure next time I brew it), but I'd be pretty confident to say it's got to be < 4.0 (if not lower), and S-23 does just fine...