Their arrival here is planned for the 2014 Detroit Auto Show starting with the M235i coupe (the coupe and convertible versions of what we know now as the 135is, pictured), and when that happens a report in Car and Driver says M Performance models will replace the "is" trim as the next-generation BMWs are being introduced. That would mean cars like the 335is become the M335i sedan and M435i coupe and convertible. Initial M flavors right now are expected to include the Z4, 1, 2, 3 and 4 Series.
The original announcement from BMW claimed that the M Performance vehicles would receive "exclusive engines," but the C/D report suggests that might not be the case. The mag also says that we'll see the M335i or M435i as soon as next year.