I've been using Arch for four or five years now. My current install has been around for three years, x86_64. In my experience, any software carried on i686 that built on x64 has always been in x64, though it's true that i686 used to get new builds first -- that has changed now and x64 usually gets new packages before i686.

Honestly I wouldn't be surprised to see i686 dropped completely in the next 1-2 years.