There is the (admittedly somewhat controversial) case of the Hawai'ian
Rock Wallaby.

75 years ago three rock wallaby's were released into the wilds of
O'ahu.  Now a colony of rock wallaby's live in Kalihi.  They cannot
interbreed with the original Australian rock wallaby's, (or presumably,
any other species) which makes them, by definition, a new species.  Time
for speciation: 75 years.  If this is finally accepted by the scientific
community, it will be the first observed case of speciation.

I'll dig up a link if anyone is interested, but I suppose that's enough
to search on if you want.

Erik