If 175hp is the target for the production motor, then it makes absolutely no sense whatsoever to arbitrarily keep this engine out of the 3 Series. Nor, for that matter, does it make any sense to forgo it for any other vehicle series where this level of power is currently as good as or better than the norm at the low end. This would cover all *14i, *16i, and *18i vehicles, which means 1 series, 3 series, and X1.
Thus, I would be very surpised if BMW's intentions are anything other than to fully replace both the current N18B16 (Mini 1.6L) and N13B16 (BMW 1.6L) four cylinder engines with this new 1.5L I3. To not do so would completely undermine the gains made by the increased economies of scale.
Furthermore, especially if the 225hp version in the I3 is accordingly efficient, I would not be surprised to see this engine additionally take over the *20i role from the current N20B20. After all, the current *20i models are only at about 184hp. Even if BMW intends to raise that to around 200hp in the coming years the I3 would still suffice. And for that matter, today's *25i N20B20 offerings (just the F20/F21 125i for now, but possibly more soon) could theoretically transition to this engine as well.
Jason or southlight - any idea what the code is for this engine? N10 perhaps?