Mind that roadsters are more sporty performance models, while cabrios are more luxury performance vehicles. So, it is much more important to keep roadster weight down & balanced - especially when BMW are going to attack Boxster / Cayman duo more seriously with the new Z4 / Z6.

3er cabrio metal folding roof fits more to luxury position of 3er. So will F13 6er cabrio - even E66 was set to get metal folding roof, yet suppliers weren't able to design a roof to fit in E66's back (no much place due rear axle design). Now F13 is designed to get a metal folding roof.

Regarding Z8: since it will be BMW's top sporty performance vehicle it is planned to get tour-de-tech special lightweight hard top folding roof. Yet there are some technical & even cost omissions - despite Z8 will be a cost car. In the case that engineering of this roof will fail (as it was the situation in E66 case), Z8 will get a canvas roof, and a Z8 coupe version.

Are you sure????

My sources told me a different story: Z4 definitely getting a canvas roof, and a Z4 Coupe twin (like in E85 / E86 case) - because this was the only way to match Boxster / Cayman driving dynamics.

well looking twice at the roof in the pic, scott's idea is plausible, the roof does seem too bulky for simply canvas, can it be metal wrapped in a canvas?

Not any more - The success of the SLK has spurred BMW to introduce it's "Mid" range Roadster with such a feature. It's no secret that soft-tops (like the Z4) do not sell as well as the folding hard-top rivals in some markets so BMW on the behest of marketing and consumer demand have put forward their plans to recooperate their market share in this segment. BMW will again introduce a Coupe model the folding hardtop will be lightweight steel similar to the 3er Cabrio , The reason why the car is out early is that the engineers want to completely change the perception of such a car and make it dynamically superior The car has been submitted a very challenging dynamics programme.

- when this car is replaced the Boxster will be well on it's way towards it's
3rd generation model

So what does the timeline for this look like Scott?

It's coming in 2nd half of 2009.

Regarding the metal folding roof: looking at the pics the cut lines of boot lid make me think metal folding roof could be there (also E93 3er was first tested with a canvas roof). But eg. E66 6er also has similar boot lid cut lines, and a very similar canvas roof (side view).

Yet my sources told me Z4 roadster successor will definitely come with a canvas roof. I'm a bit confused now.

I personally would never want a metal folding roof on any sort of sports car. Thats so much extra weight, and they usually look bad, buy the coupe version if you want a metal roof.

I agree with you.

Clues :
Side Window - More found on folding top roadsters. 4 seater cabrio it can work but it does look severely out of place on a 2 seat roadster.
Rear deck - You can see the opening in the camouflage for the roof - similar to the E93.
The folding roof will not be as "complex" as the 3er just a simple Folding top.
Canvas roof - No harmony or shape looks much more bulkier than current Z4 top .

Deep down you know what it will turn out like...

The top end engine for the Z4 will be the 335i's twin-turbo tweaked further until the M car comes along in 2010/2011 - That would probably receive the upcoming M3's V8.
For now that engine remains exclusive to the 335i in europe only . Unlike the USA which will get a 535i .