I may be wrong but I don't think MB has a 3.5 liter V-6. The 350 engines are really 3.7 liter V-6's. Someone may know something I don't.

Have you ever looked at a 300M? They reek of S-Class similarities! 

Well, I can confirm that the M272 (Mercedes 3.5L) is actually based upon the M112 (Mercedes 3.2L), which dates back to at least 1997. I owned a MY1998 CLK320 nearly 125K miles and never had a single mechanical or electrical problem with the 3.2 -- I suspect the quality figures MB get dinged for aren't about major mechanical or construction issues.

I recently took delivery of a 2006 SLK350 (with the 3.5L M272), and went looking for as much information as I could find. You can see a training PDF for the engine posted at http://www.billsden.org/mercedes/slk/docs/train_engine_m272.pdf. If you were looking for telltale signs that the 3.5 really is Mercedes engine and not just "M112 based" for convenience, I'd point you to the detail about the oil level sensor. The M112 (and it's V8 stablemate) were capable of sensing and reporting the actual oii level in the engine as well as the clarity of the oil (which was part of the FSS calculation). The M272 removes this capability -- it was apparently confusing to customers in general, but I always liked it... -- but didn't physically remove the switch piece from the engine casting. Instead they just retired all but one of the leads connected to it.

I don't know where the Chrysler 3.5 comes from, but I suspect these are not the same engine. On a more visceral level, if Chrysler and MB started drivetrain or chasis swapping, I'd have to think long and hard about my next automobile purchase. After all, do I really want a MB that's just an upscale Chrysler in the same way that several Infiniti, Lexus, Audi, and Jaguar models are upscale from other makes?

I would point out that BMW have smartly started advertising along these lines -- brand and corporate purity -- but sadly try to pass off DaimlerChrysler -- why oh why did they ever change the name? -- as another of these rebranding arrangements. As much for Chrysler's sake as Mercedes', these lines need to remain as distinct as possible in order to remain relevant in the marketplace. In most of these rebranding arrangements, you can get 80-90% of the car for 60-70% of the price... where's the drive to upscale? How does one ride around in them without wondering why they spent more to get a very similar car?

But let's put something else to bed. It was Daimler-Benz AG, not Mercedes, that acquired Chrysler. DBAG and Mercedes were not one and the same, since DBAG holds several companies. Chrysler was just another on the pile. I suppose this was part of the reason they insisted on renaming DBAG as part of the deal, but I think in the long run it just generated more confusion than anything. It's still a German AG that holds an American subsidiary. 

Just look at what is happening in the hybrid and fuel cell space. We see the formation of apparently unholy alliances because those who haven't bothered investing are desperate and need to align themselves with those who are better prepared. I guess what this results in is general confusion of how cars that start sharing key components (note, not "parts") can really differentiate. For some, what makes a car special isn't just a few body panels, some leather, and a few splashes of wood and chrome... but instead something more to do with it's design, engineering, and craftsmanship from the bottom up. And, frankly, that's why the American brands still get their ass handed to them on a regular basis. They lost what made them special in their misquided attempts to remain "competitive."