Having recently feted the arrival of the new M5, BMW M fans have another reason to celebrate with the announcement of an all-new, Z4-based M roadster. The last iteration, spawned from the Z3, impressed us with its raw speed, but it lacked sophistication, and the styling was more retro-cute than muscular. This newest version will have to offer much more in the way of panache and refinement if BMW hopes to mount a credible threat to the stellar-and recently updated-Porsche Boxster S. The BMW will be armed with the 3.2-liter in-line six from the departing E46 M3. Other changes from the standard Z4 include the addition of BMW's variable M-differential lock, a stiffer suspension, and a more traditional hydraulic-rather than electric-power steering setup. Cosmetically, the M roadster gains revised front and rear fascias and eighteen-inch wheels. The car debuts at the Detroit auto show and should go on sale in the summer.