Everywhere I've lived in North America (Texas, Alberta, Maryland, Washington DC, New York, and California), people have pronounced subtle in a way that rhymes with scuttle. It occurred to me, though, that some people might pronounce the now rare (in U.S. English) word subtile differently.
In checking Merriam-Webster's Eleventh Collegiate Dictionary (2003), I was surprised to discover that Merriam-Webster's does not consider subtile to be a variant of subtle; instead, it dates both terms to the 14th century, and provides each one with its own full entry and its own complement of definitions (many of them quite similar):
subtile 1 : SUBTLE, ELUSIVE [a subtile aroma] 2 a : CUNNING, CRAFTY b : SAGACIOUS, DISCERNING
subtle 1 a : DELICATE, ELUSIVE [a subtle fragrance] b : difficult to understand or distinguish : OBSCURE [subtle differences in sound] 2 a : PERCEPTIVE, REFINED [a writer's sharp and subtle moral sense] b : having or marked by keen insight and ability to penetrate deeply and thoroughly [a subtle scholar] 3 a : highly skillful : EXPERT [a subtle craftsman] b : cunningly made or contrived : INGENIOUS 4 : ARTFUL, CRAFTY [a subtle rogue] 5 : operating insidiously [subtle poisons]
With regard to your question about pronunciation, Merriam-Webster's offers only a no-b pronunciation of subtle, but it includes a voiced-b secondary pronunciation for subtile (the primary pronunciation it gives for subtile is identical to the one it gives for subtle). I have no idea how Merriam-Webster's arrived at its conclusion that a significant number of U.S. English speakers pronounce subtile with a voiced b.
In a Google Books search for works published between 1900 and 2000 that contain the word subtile, most turn up in the context of reprints or quotations from authors writing in previous centuries. The few truly modern exceptions chiefly involve a new word subtile (presumably pronounced sub-tyle) that refers to the underside of special heat-resistant tiles affixed to aircraft and spacecraft in aeronautics.