The Speedster will probably be offered at the end of the 987.2 S production cycle as the RS60 was offered towards the end of the 987 S production.  In other words, it will probably be offered just before the introduction of the newly designed 2011 988, sometime in 2010.    

I would also guess that Porsche will offer the 987.2 Speedster with the Cayman 320hp engine to distinguish it from the regular 987.2 S 310hp offering, and charge a premium over the Cayman S for the extra body work.    It would be a nice package.

vincesf